How exclude a certain update from installing?

How shall I enter that in Exclude > Custom?
In an exludelist.txt? As KBxxxxxxx or just xxxxx?
Or directly as KBxxxxxxx or just xxxxx?
Or does that only exclude it from being downloaded into WSUS instead of from installing it when updating?
In an exludelist.txt? As KBxxxxxxx or just xxxxx?
Or directly as KBxxxxxxx or just xxxxx?
Or does that only exclude it from being downloaded into WSUS instead of from installing it when updating?