The calculation rule XPPF is used to calculate the wage types /801 and /802, which are the gross and net pay for the current period. The operation XPPF calls the personnel calculation rules XPPO and XPP1, which perform the following steps:
XPPO: This rule calculates the gross pay (/801) by adding up all the wage types in the IT table that have the processing class 10 set to 1. The operation ADDWT&10 adds the wage types to the /801 wage type and stores it in the OT table. The operation ADDCU cumulates the /801 wage type in the CRT table.
XPP1: This rule calculates the net pay (/802) by subtracting the deductions from the gross pay. The operation SUBWT&11 subtracts the wage types in the IT table that have the processing class 11 set to 1 from the /801 wage type and stores the result in the /802 wage type in the OT table. The operation ADDCU cumulates the /802 wage type in the CRT table.
References = Payroll Schema, SAP Certified Application Associate - SAP HCM Payroll for SAP S/4HANA, SAP HCM Payroll for SAP S/4HANA - SAP Training Shop, Internal Tables for Payroll

In the SAP HCM Payroll for SAP S/4HANA, the function PIT (Process Input Table) uses parameters to define its behavior. Parameter 2 is used to specify the type of access for the personnel calculation rule. The allowed values for parameter 2 in the PIT function are 'GEN' and 'NOAB'. 'GEN' stands for Generic access of personnel calculation rule (process any wagetype). 'NOAB' stands for Employee subgroup grouping personnel calculation from table employee group/employee subgroup (T503) without payroll type (Process for any EE sub-group groupings).
References =
https://answers.sap.com/questions/8719427/reg-parameters-in-schema.html
https://community.sap.com/t5/enterprise-resource-planning-blogs-by-members/understanding-functions-in-payro

Wage types in SAP HCM Payroll are used to assign payments and deductions as well as to control the payroll program. They are the building blocks of the payroll module and areessential to compensate the employees1. In the case of an hourly wage type, the hourly rate, which should be multiplied by the number of hours performed by the employee, during the payroll run2. Therefore, wage types /001 and /002 can be used to calculate the hourly base rate for salaried employees. References =
https://help.sap.com/docs/SAP_S4HANA_ON-PREMISE/e533a9d0e7e547818964b2fd3226924b/953ac2531bb9
https://help.sap.com/docs/SAP_S4HANA_ON-PREMISE/21a0a960d9b44ccaa6ac02f87e100f2f/37cade5371874

In the payroll period in which the absence begins (B): The system compares the cut-off day with the start date and the end date of an absence record1. If the cut-off day is later than the end date, the system continues with the standard absence valuation process1.
On the actual dates in each affected payroll period : If the cut-off day is earlier than the start date, the system transfers the absence to the corresponding carry-over wage type, which is saved in an internal table for absence valuation in the next payroll period1.
In the payroll period in which the absence ends (E): If the cut-off day is between the start date and the end date, the system splits the absence into two records: One record is from the start date to the cut-off day, for which the system continues with the standard absence valuation. The other record is from the cut-off day to the original end date, which the system saves in the carryover wage type and will evaluate in the next payroll period1. References =
https://help.sap.com/docs/SAP_S4HANA_ON-PREMISE/7aff27bde57a4bd7b89e8cce540e110e/dede391612cc4

When a personnel number is rejected in the payroll run, the system sets the Payroll Correction indicator in infotype 0003 (Payroll Status) for that employee1.
When a personnel number is subject to master data changes in the correction phase, the system also sets the Payroll Correction indicator in infotype 0003 (Payroll Status) for that employee2.
The Payroll Correction indicator means that the payroll results of the employee need to be recalculated in the next payroll run3.
Therefore, the system uses the Payroll Correction indicator in infotype 0003 (Payroll Status) to identify the personnel numbers whose payroll results need to be recalculated in the next payroll run. References =
1: SAP Help Portal, Payroll Control Record, section "Rejected Personnel Numbers".
2: SAP Help Portal, Payroll Control Record, section "Master Data Changes in the Correction Phase".
3: SAP Help Portal, Payroll Control Record, section "Payroll Correction".
4: SAP Help Portal, [Payroll Control Record].