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STOCK-BASED COMPENSATION (Tables)
 12 Months Ended
Dec. 31, 2021
Stock-Based Compensation [Abstract]  
Components of Stock-based Compensation Expense
The following table summarizes the components of stock-based compensation expense included in the consolidated statements of operations:

 
  
Year Ended December 31,
  
 
  
2021
    
2020
    
2019
  
Stock-Based Compensation Expense:
                  
Cost of revenue
  
$
397
    
$
175
    
$
250
  
Sales and marketing expense
    
1,802
      
1,199
      
328
  
Research and development expense
    
1,115
      
763
      
1,093
  
General and administrative expense
    
1,803
      
3,491
      
1,901
  
 
  
$
5,117
    
$
5,628
    
$
3,572
  
 
 
  
Year Ended December 31,
  
 
  
2021
    
2020
    
2019
  
Stock-based compensation expense by type:
                  
Employee stock purchase plan
  
$
4,674
    
$
4,900
    
$
2,265
  
Non-employee stock purchase plan
    
94
      
396
      
1,307
  
Subsidiary option grants
    
349
      
332
      
-
  
 
  
$
5,117
    
$
5,628
    
$
3,572
  
Assumptions Used to Determine Fair Value of Share Options Granted
The fair value of options granted to employees with a service period based condition is estimated on the grant date using the Black-Scholes valuation model with the following assumptions:

   
Year Ended December 31,
  
   
2021
    
2020
    
2019
  
Fair value of common share(1)
  
$
38.38-51.07
    
$
22.07-85.27
    
$
13.64-16.81
  
Expected term in years(2)
    
6.25
      
5.50-6.25
      
6.25
  
Volatility(3)
    
48.53-49.47
%
    
42.17%-48.15
%
    
39.91%-40.35
%
Risk-free interest rate(4)
    
1.00%-1.44
%
    
0.44%-0.82
%
    
1.69%-2.46
%
Expected dividend(5)
    
0
%
    
0
%
    
0
%
 

 (1)
Fair value of Class A common stock value was the closing market price of the Class A common stock on the grant date.

(2)
Expected term of share options is based on the average of the vesting period and the contractual term for each grant according to Staff Accounting Bulletin 110.

(3)
Volatility is calculated based on the historical volatility of the stock of companies comparable to ACM in the period equal to the expected term of each grant.

(4)
Risk-free interest rate is based on the yields of U.S. Treasury securities with maturities similar to the expected term of the share options in effect at the time of grant.

(5)
Expected dividend is assumed to be 0% as ACM has no history or expectation of paying a dividend on its common stock.
 
During the year ended December 31, 2020, the fair value of option granted to an employee with market based condition was estimated on the grant date using the Monte Carlo simulation model with the following assumptions:
 

  
Year Ended
December 31,
  
   
2020
  
Fair value of common share(1)
  
$
22.07
  
Expected term in years(2)
    
9.20 - 9.80
  
Volatility(3)
    
45.10
%
Risk-free interest rate(4)
    
2.68
%
Expected dividend(5)
     0
%
 

 (1)
Fair value of Class A common stock value was the closing market price of the Class A common stock on the grant date.

(2)
Expected term of share options is based on the average of the vesting period and the contractual term for each grant according to Staff Accounting Bulletin 110.

(3)
Volatility is calculated based on the historical volatility of the stock of companies comparable to ACM in the period equal to the expected term of each grant.

(4)
Risk-free interest rate is based on the yields of U.S. Treasury securities with maturities similar to the expected term of the share options in effect at the time of grant.

(5)
Expected dividend is assumed to be 0%, as ACM has no history or expectation of paying a dividend on its common stock.
Employee Share Option [Member]  
Stock-Based Compensation [Abstract]  
Summary of Share Option Activities
The following table summarizes the Company’s employee share option activities during the years ended December 31, 2019, 2020 and 2021:

 
Number of
Option Share
    
Weighted
Average Grant
Date Fair Value
    
Weighted
Average
Exercise
Price
  
Weighted Average
Remaining
Contractual Term
Outstanding at December 31, 2018
    
2,503,405
    
$
0.91
    
$
4.09
  
7.30 years
Granted
    
656,000
      
6.29
      
16.21
    
Exercised
    
(106,768
)
    
0.60
      
2.09
    
Expired
    
(2,757
)
    
3.34
      
8.16
    
Forfeited/cancelled
    
(55,817
)
    
2.38
      
6.23
    
Outstanding at December 31, 2019
    
2,994,063
      
2.59
      
6.77
  
7.05 years
Granted
    
786,399
      
12.17
      
29.17
    
Exercised
    
(547,189
)
    
1.34
      
3.78
    
Forfeited/cancelled
    
(41,862
)
    
4.80
      
12.65
    
Outstanding at December 31, 2020
    
3,191,411
      
5.13
      
12.73
  
7.13 years
Granted
    
140,400
      
48.16
      
106.15
    
Exercised
    
(477,058
)
    
2.46
      
6.30
    
Forfeited/cancelled
    
(54,004
)
    
24.97
      
57.10
    
Outstanding at December 31, 2021
    
2,800,749
    
$
7.36
    
$
17.65
  
6.53 years
Vested and exercisable at December 31, 2021
    
1,922,180
                        
Employee Share Option [Member] | ACM Shanghai [Member]  
Stock-Based Compensation [Abstract]  
Summary of Share Option Activities
The following table summarizes the ACM Shanghai employee stock option activities during the years ended December 31, 2021 and 2020:

 
Number of
Option Shares in
ACM Shanghai
    
Weighted
Average Grant
Date Fair Value
    
Weighted
Average
Exercise
Price
  
Weighted Average
Remaining
Contractual Term
Outstanding at December 31, 2019
    
-
    
$
-
    
$
-
  
-
Granted
    
5,869,808
      
0.23
      
1.89
    
Exercised
    
-
      
-
      
-
    
Expired
    
-
      
-
      
-
    
Forfeited/cancelled
    
(446,154
)
    
0.23
      
1.89
    
Outstanding at December 31, 2020
    
5,423,654
    
$
0.23
    
$
1.89
  
3.50 years
Granted     -       -       -    
Exercised     -       -       -    
Expired     -       -       -    
Forfeited/cancelled     (46,154 )     0.24       2.04    
Outstanding at December 31, 2021     5,377,500     $ 0.24     $ 2.04   2.50 years
Vested and exercisable at December 31, 2021
    
-
                    
Non-Employee Stock Option [Member]  
Stock-Based Compensation [Abstract]  
Summary of Share Option Activities
The following table summarizes the Company’s non-employee share option activities during the years ended December 31, 2019, 2020 and 2021:

 
Number of
Option Shares
    
Weighted
Average Grant
Date Fair Value
    
Weighted
Average
Exercise
Price
  
Weighted Average
Remaining
Contractual Term
Outstanding at December 31, 2018
    
1,212,374
    
$
0.78
    
$
 2.57  
6.66 years
Granted
    
-
      
-
      
-
    
Exercised
    
(88,529
)
    
0.45
      
1.06
    
Expired
    
-
      
-
      
-
    
Forfeited/cancelled
    
(22,232
)
    
0.55
      
3.00
    
Outstanding at December 31, 2019
    
1,101,613
      
0.82
      
2.69
  
5.85 years
Granted
    
20,000
      
10.29
      
25.60
    
Exercised
    
(285,315
)
    
0.88
      
3.17
    
Expired
    
-
       -       -      
Forfeited/cancelled
    
(260
)
    
0.30
      
0.75
    
Outstanding at December 31, 2020
    
836,038
      
1.02
      
3.07
  
4.92 years
Granted
    
-
      
-
      
-
    
Exercised
    
(146,543
)
    
1.12
      
3.83
    
Expired
    
-
      
-
      
-
    
Forfeited/cancelled
    
(489
)
    
0.34
      
0.84
    
Outstanding at December 31, 2021
    
689,006
    
$
1.00
    
$
2.91
  
3.98 years
Vested and exercisable at December 31, 2021
    
677,756