OK, I'm kind off a nation modder, I'm working on several projects you might see sometime soon, this is a helpful guide I made so you don't have to go crazy looking for these numbers, I'll explain:
In the data/unitrules.xml file, there are certain 24 character numbers which allow NATION X to have UNIT X, for instance, the Armored Car unit has 24 number 1s, meaning it's enabled for every single nation (each number represents one nation), and the Inca Inti Macemen unit has 23 ZEROES (meaning it's only avaibable to them) and a single 1, in this case located in the 22nd position (the Inca's slot) so if your making a unit you can specify which nations get it, and which don't, this guide provides every nation's "slot" (INCA: 22nd; Aztecs: 24th and so on)
Made on January 29th by Montero.