God damn I hate the loving GECK, holy forget. Why the forget does it let me load multiple ESP files if I can't actually reference any of them in the active file? You can force an ESP as a parent master with FO4Edit, but when you try to edit it in GECK it just loving wipes the parent masters that aren't ESMs and makes a thousand stuffty duplicates. This means you can't download a weapon mod, and then use that weapon for a custom NPC in another mod.
Or at-least I can't figure it the forget out. If it's possible the documentation is worse than Blockland's and everyone on the Nexus forum is a dumb forget.