In mempool dump through getrawmempool true, I see something called modifiedfee.

It seems to be the same in all cases (just by looking at the dump). What exactly is it, how does it relate to fee (that is also present)?

The help says

(numeric) transaction fee with fee deltas used for mining priority

but that is not very helpful; that just seems like normal fee.

edit: I went trough the file by iterating over the dump, and modifiedfee and fee are indeed the same on all the transactions.