Hello @antoniott15,

Thanks for your question. Happy to help!

The Start event is the event a user must perform in order to be included in the Retention Analysis, and then be analyzed for the Return event. 

The Return event is the event a user must perform in order to be counted as retained.

Amplitude computes retention data by comparing the date of that starting event to the date of the return event you’ve specified.

I am not quite sure I fully understand the latter half of your question, but what I can say is that simply filtering on an event in the user segment will not give you a calculation of retention. Amplitude requires a Start and Return event in order to provide a calculation for retention.

I have also linked our Help articles on the Retention Analysis chart here, for your reference.

Please do let me know if you have more questions!