What is "Driver Level" and how is it determined?

So we have Collector Level, we have League Levels and Bands, we have the hidden True Skill ranking.

Maybe I am old - it seems to be a term that shows up on Forum searches from a few years ago - FM3-4 time frame.
On the main menu select ACHIEVEMENTS, Then select STATS on the left bar

Stats and rank among friends are displayed for these four metrics
Time Played, Collector Tier, Cars Owned and Driver Level

There is a numerical score exhibited for Driver Level. What is that and how is it determined? - Darned if I can remember!

It’s basically how many miles you’ve driven.

It’s XP earned (by number of races, race length, mods)