Employee Subgroup Groupings for PCRs:This setting allows you to categorize employees into groups to apply different payroll rules. SAP reserves values below 10 (00 - 09) for its use.
Customer-Specific Values:You can use the range 00 to 99, excluding the fixed SAP values of 5 to 9, to create your custom groupings. This flexibility allows you to tailor payroll rules according to your company's specific needs.

The function that uses MODIF A to evaluate absences in SAP is RAB1. This operation is part of the payroll process and is used to control access to the table Absence Valuation during absence valuation. MODIF A specifies which employee subgroup groupings in which table entries the system must take into account for absence valuation1.

Secondary wage types are technical wage types that are created during payroll run and start with a slash (/).
They are used for various purposes such as valuation, calculation,and reporting. For average calculation, which is a method of valuing wage types according to the principle of averages, the system uses the following secondary wage types:
/201 Average basis 01 (Time wage earner): This wage type stores the amount of the wage types that are relevant for average calculation for time wage earners.
/202 Average basis 02 Premium amount: This wage type stores the amount of the wage types that are relevant for average calculation for premium payments.
/203 Average basis 03 (Incentive wage earner): This wage type stores the amount of the wage types that are relevant for average calculation for incentive wage earners.
/210 Average posit. time (day): This wage type stores the positive time in days that are relevant for average calculation.
/211 Average posit. time (hour): This wage type stores the positive time in hours that are relevant for average calculation.
/212 Average neg.time (day): This wage type stores the negative time in days that are relevant for average calculation.
/213 Average neg. time (hours): This wage type stores the negative time in hours that are relevant for average calculation.
/216 Gratuity time (day): This wage type stores the gratuity time in days that are relevant for average calculation.
/217 Gratuity time (hours): This wage type stores the gratuity time in hours that are relevant for average calculation.
/240 Average wage dialog (day): This wage type stores the value of the average wage due per day multiplied by 100. It can be used to override the incentive wage earners' daily average in the infotype
0008.
/241 Average wage dialog (hrs): This wage type stores the value of the average wage due per hour multiplied by 100. It can be used to override the incentive wage earners' hourly average in the infotype
0008.
/242 Aver.diff. (day) * 100!!: This wage type stores the value of the average wage difference (DIFF) due per day multiplied by 100. It can be used to override the time wage earners' daily average in the infotype 0008.
/243 Aver.diff. (hour) * 100!!: This wage type stores the value of the average wage difference (DIFF) due per hour multiplied by 100. It can be used to override the time wage earners' hourly average in the infotype 0008.
/250 Average value (Ft/day): This wage type stores the calculated value of the daily average for the reference period. It is used for evaluation with the daily average (e.g. for printing in the remuneration statement).
/251 Average value (Ft/hour): This wage type stores the calculated value of the hourly average for the reference period. It is used for evaluation with the hourly average (e.g. for printing in the remuneration statement).
/260 Difference comp. in payr.: This wage type stores the value of DIFF * NUM. It is used for consideration of wage types evaluated with average in bases for calculating average value.
/261 Absence fee difference: This wage type stores the value of DIFF * NUM. It is used for consideration of wage types evaluated with absence fee in bases for calculating average value.
References = Wage types of average calculation

In SAP HCM Payroll for SAP S/4HANA, the operation TSDIVP (Time Schedule Division Period) is used to query the average working hours per pay period for each partial period in table PARTT. This operation calculates the average working hours by dividing the total working hours by the number of pay periods. It takes into account the specific time schedule and partial periods to provide an accurate calculation.
References =
https://stackoverflow.com/questions/35907459/how-to-get-the-total-working-hours-for-employees-with-sql-serv
https://stackoverflow.com/questions/20004156/hours-and-average-hours-worked-per-day-by-department