The first one will be 01, next should be 02, and so on. The list below is in order of which DLC will load first (and also in order of release).

If you have mods installed which are loaded before the DLC packs, their numbers will be different entirely (higher). If you have only The Pitt installed, IDs for its objects will start with '01'. 010018d2) and IDs for objects from The Pitt will start with '02' (e.g. from Operation: Anchorage will start with '01' (e.g. If you have both Operation: Anchorage and The Pitt installed, IDs for characters, objects etc. The only 'fixed' IDs are those from 'base' Fallout 3, Fallout: New Vegas and Fallout 4 games (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').

The first two digits of a Form ID correspond to its load order. This is because the load order of modules (ESMs and ESPs) affects those numbers. Originally posted by Fallout Wiki: Form IDs from add-ons start with 'xx' because their first two numbers vary from player to player, depending on which mods and DLC packs are installed on his/her system.

Fallout 4 item codes google docs