I think that you are mostly correct here, except I would describe it a little bit differently.
It's mempool data:
https://mempool.space/api/v1/fees/recommended{"fastestFee":14,"halfHourFee":13,"hourFee":13,"economyFee":6,"minimumFee":3}
vs
High Priority. This figure is the median feerate of transactions in the first mempool block. Consider using this feerate if you want confirmation as soon as possible.
Medium Priority. This figure is the average of the median feerate of the first mempool block and the median feerate of the second mempool block.
Low Priority. This figure is the average of the Medium Priority feerate and the median feerate of the third mempool block. Consider using this feerate if you want confirmation soon but don't need it particularly quickly.
No Priority. This figure is either 2x the minimum feerate, or the Low Priority feerate (whichever is lower). Consider using this feerate if you are in no rush and don't mind if confirmation takes a while.
And yeah, you're right, the last one is the purging limit on default nodes.
