Hi @Kirankumarbs 
Please let me know if my understanding is correct 🙂
So when it tries to read source table it matches the src schema in checkpoint with the current source schema and add on changes as new changes which are still not part of checkpoint schema 
Suppose If I have added a new column in initial stage then it would do 2 level of comparison
1. source schema
2. new cols
So it does comparison step by step by replaying the previous steps in the version N.