Well, it’s not free actually but bound to the (disk on the) device and being paid for with the purchase of the device. As long as you see a running routerOS when it’s booted, you’re safe.There’s no recurring cost.
-Chris
Check the license levels table to see the difference between level 3 and level 4 (which nowadays most RB devices come with). The only big difference is that it’s not possible configure a level 3 device as AP, it can only be client. In your PtMP scenario it can be one of “rim” elements but it can’t be a “hub” (speaking in bicycle wheel nomenclature).