The files you need to check:
Regular Units
...\mods\SS6.3\data\export_descr_buildings.txt
For each recruitment building you will see entries like:
Code:
recruit_pool "Imperial Knights" 1 0.1 1 0 requires factions { hre, } and event_counter FULL_PLATE_ARMOR 1 and not event_counter GOTHIC_ARMOR 1
That means Imperial Knights are available after the "full plate armor" event until the "gothic armour" event.
Mercenaries
...\mods\SS6.3\data\world\maps\campaign\imperial_campaign\descr_mercenaries.txt
For each region you will see entries like:
Code:
unit Mercenary German Knights exp 1 cost 895 replenish 0.04 - 0.13 max 2 initial 1 start_year 1380 religions { catholic orthodox }
That means Mercenary German Knights are available in that region after 1380 only for catholic factions.
If you want to know after how many years events are triggered, open ...\mods\SS6.3\data\world\maps\campaign\imperial_campaign\descr_events.txt