Ambiguous clear_notification note for iOS

[kristian]

The note in the Clearing section mentions for iOS:

will only clear the most recent critical notification from a given tag.

This statement is ambiguous, does it mean:

a) In case the level of the notification is critical, it will clear the most recent or
b) The feature to clear the notification only works for critical notifications in the first place

I am sending a passive notification and somehow sending a clear_notification message doesn't seem to be doing anything. Now I am unsure if clearing only works for critical messages (so b) ), or its something other I do wrong, so the notification isn't cleared?