org-agenda-category-filter-preset

org-agenda-category-filter-preset is a variable defined in `org-agenda.el'.
Its value is

nil

Documentation:
A preset of the category filter used for secondary agenda filtering.
This must be a list of strings, each string must be a single category
preceded by "+" or "-".
This variable should not be set directly, but agenda custom commands can
bind it in the options section. The preset filter is a global property of
the entire agenda view. In a block agenda, it will not work reliably to
define a filter for one of the individual blocks. You need to set it in
the global options and expect it to be applied to the entire view.