Why do Form IDs from mods start with "xx"?Edit
Form IDs from mods start with "xx" because their first two numbers vary from player to player, mods and DLC packs are installed on his/her system.
This is because the load order of modules (ESMs and ESPs) affects those numbers. The first two digits of a ID number correspond to its load order (in hexadecimal). The only "fixed" IDs are those from "base" Fallout 3 (which always start with "00" because it is always loaded first) and those for objects which are/have been created during gameplay such as spawned characters or items (those IDs always start with "FF").
How to know what replaces "xx" on your systemEdit
When lauching the game with FOMM (FallOut Mod Manager), the GTS - Iceland will have a "mod index", this is what "xx" will replace ingame.