How On Eligibility Option for Grade Step Transaction Date works

Hello,

we are considering to use the On Eligiblity' transaction date for our requirement, but have some clarifications on the process specifically in this 'Have the 'Process Decide the Period.

We have this requirement to get those assignments that will be progressing 15 days ahead. We are wondering if, given the example, We want to run the process every 2 weeks. Suppose we have this employee that will progress on August 10. So if the process runs on August 15, should the assignment from August 10 will have the Effective Start Date of August 15? or it can still get the Effective Start Date of August 10 which is the date the she should progress?