BuddyPress.org

In this context, item_id is related to the component.
For example, if the component is ‘groups’, then the item_id is a group id.
And secondary_item_id is kinda like another filter, maybe a comment_id.

But custom-rolling an activity list is going to lead you, at some point, into a dead end.

A much better approach is to use the many parameters of bp_has_activities.
And maybe write some custom sql to gather data arrays to feed into that function.