Hello Anshul,
The database’s history is represented as a collection of snapshots and logs that create the Timeflow.
You need time-flow point parameters, to provision VDB from any point in time between snapshots.
From Oracle perspective, FirstChangePoint is a start of RMAN process, lastChangePoint is the last SCN of RMAN backup and Delphix need archive logs between to made backup consistent.
Hope this explains.
~Regards
Jatinder Luthra