Seniority date Screen fields

Summary: Oracle has delivered below items for seniority date

EFF_ST_DATE, SENIORITY_ADJUST_COMMENT, SENIORITY_ADJUST_IN_DAYS, SENIORITY_ADJUST_IN_HOURS

In fast formula, we are not passing any variables or data to these oracle variables but it is getting some data. Can someone please explain how these fields are helping to page "seniority date screen fields"?

Seniority Date

Length of Service

Seniority-Change Start Date

Seniority-Change End Date

Business Title

Seniority Basis

Enterprise Start Date

Legal Employer

Worker Type

Assignment Number

if we don't create any rule(fast formula) then on what base, system is calculating the seniority date screen fields and reflecting values for employees.