Tag
From Crusaderkings
A realm's TAG is the four character marker used to identify it by the CK program and saved game.
For counties, it is a C followed by three digits counting more or less from northwest to southeast across the map. A complete list can be found here or in your game's config/province_names. A map of county tags can be found here.
For duchies and kingdoms, the tag is usually (but not always) the first four letters of the realm in question. A partial list can be found here; the rest are here. Their in-game display is controlled by config/world_names and config/extra_text.
Tags are all hardcoded to be at a certain level. You can edit a scenario or a saved file to make a kingdom act like a county for awhile or vice versa, but eventually things will right themselves: once independent, the kingdom will remain so; the county will bevassal itself to the appropriate duchy; etc. Further, 'elevated' tags will still behave as they were, without demesne or prestige bonuses from their modded rank. A list of the hardcoded level of tags can be found here.
