Within Foundation when creating an Activity a user can specify it belongs to a Category. However this list is completely different than what the user finds in the Activity Category vocabulary. The key difference is the category is managed for two different applications. The dropdown in Hub is managed by the list in activity categories. While the vocabulary manages the categories for Compose.

To manage the Compose activity categories the user can simply add or remove categories from the vocabulary. To manage the activity categories in Hub is more complex. To change the category in Hub the user has to make the change in the activity itself. To remove a category in Hub the user must make sure said category isn't in use. 

Example: The user assigns ten Activities to the new category "Generic". To remove "Generic" as one of the Category options the user must change the Category for each of the ten Activities.

It is also important to note the Category field is free from meaning you can add anything you want as a Category.