org-icalendar-categories

org-icalendar-categories is a variable defined in `ox-icalendar.el'.
Its value is

(local-tags category)

Documentation:
Items that should be entered into the "categories" field.

This is a list of symbols, the following are valid:
`category' The Org mode category of the current file or tree
`todo-state' The todo state, if any
`local-tags' The tags, defined in the current line
`all-tags' All tags, including inherited ones.

You can customize this variable.