Item Category in Pick Release Rule

what is the usage of the item Category in Pick Release Rule? doesn't it is a filter? i tried to setup it as filter but when I did a release, the item code is not released.

setup I did:

1) create 1 item category set CS1 with 2 categories (FG, SVC)

2) assign item code A with category FG, assign item code B with category SVC

3) create 1 sales order with 2 lines, item code A, item code B. set both lines with ship set 1.

4) create 2 pick release rules. pick release rule A has item category FG, and pick release rule B has item category SVC.