There is no such thing as categories of priority. Priority is either high enough to not be treated as zero, or it's not. And everything is linear in priority after that.
(You're referring to BC.i's behavior, I think.... which has little to no connection to reality.)
Thanks for the answer. A binary graph would be easier, but I was refering to the following categories in bitcoin core/qt:
-Im shy about my coins-
I know at this stage its just an unspend output from the previous transaction, but judging how high amounts and few confirmations get a higher "rating" I though this was related to the the priority of a transaction.
I found it in the source[1] now and it looks like a log scale around the hard coded value "AllowFreeThreshold()"[2] (aka "bitcoin day") or as of 0.10. an estimate depending on the nodes mempool.
[1]
https://github.com/bitcoin/bitcoin/blob/7620ef9e7edbc3526e71922e7b080b5a332d6897/src/qt/coincontroldialog.cpp#L422[2]
https://github.com/bitcoin/bitcoin/blob/f914f1a746d7f91951c1da262a4a749dd3ebfa71/src/txmempool.h#L18