Custom items - DRE2N/DungeonsXL GitHub Wiki
DungeonsXL uses the custom item (and mob) library Caliburn which allows users to register custom items and then reference them by their ID as if they were Vanilla items.
Caliburn vs. ItemsXL
What ItemsXL mainly does is making Caliburn a standalone runnable plugin. DungeonsXL already contains Caliburn, so it works as it is without ItemsXL. However, ItemsXL contains some commands to manage Caliburn easily in game, such as commands to set up loot tables, register items and mobs.
"Why can't I use items from custom item plugin XY?"
Support for other custom item plugins will not be added even if PRed since Caliburn is being developed into a bridge between custom item system. Its role is about the same as Vault's for permission plugins.