Its pretty easy to create a category. Navigate to UltraCollections/gui and in the gui folder, simply create a new yml, for example, combat.yml. Then, paste the code below in it, you can modify this as to your needs.
The category_name is important as that is what you define in collections for them to show up in this category.
Actions
There is a actions system with which you can make certain items do certain things when left clicked, here are all the available actions (opengui, play_sound, run_command & send_message):
actions:
opengui: main
play_sound:
sound: "ENTITY_ITEM_PICKUP"
volume: 1.0
pitch: 1.0
send_message:
message: You have not unlocked this yet!
run_command:
command: crafting open dirt_minion # example command
as_op: true