According to Oracle HCM documentation12, Payroll is the module within HCM that is used to process the benefits selected by the employee, which will ultimately affect the employee's net pay.
Payroll is a system that calculates and distributes payments and deductions for employees based on their compensation and benefits. Payroll also handles tax reporting, third party payments, net-to-gross processing, and other payroll-related functions12
https://docs.oracle.com/cd/E18727_01/doc.121/e13533/T4895T4900.htm

The Pending Worker Self Register process allows workers to register themselves as pending workers in the system. This is done through the use of self-registration forms that are sent to applicants or other potential workers. When the form is completed and returned, Oracle HCM processes the information to create a pending worker record. This allows the workers to have access to the system and complete the onboarding process without the intervention of an HR representative.
https://www.oracle.com/webfolder/technetwork/tutorials/obe/fusionapps/HCM/WorkerRestAPI_CreateWorker/html/index.html

Workforce Management is a fully integrated HR solution that links time, labor, and leave management with payroll, financial, and personnel data.
Employee Separation process is to reconcile absence records, which would involve managing the leave balance and entitlements of the departing employee.
https://www.oracle.com/human-capital-management/workforce-management/

According to Oracle HCM Business Process Foundations Associate Rel 1, other than the employee, the HR Specialist and Line Manager can manage the employee's absence records. This includes entering, modifying, and approving absence information.
https://support.oracle.com/knowledge/Oracle%20Fusion%20Applications/1998783_1.html

Explanation
According to the Oracle HCM Business Process Foundations Associate Rel 1 official book, the four groups of compensation history are Recurring Payments, Other Compensation, Salary, and Payroll. Recurring Payments include items such as bonuses, commissions, and performance-based payments. Other Compensation includes items such as allowances, reimbursements, and vacation pay. Salary is the base pay for an employee. Payroll includes items such as taxes, deductions, and contributions.