Attachment combs are put on top of the nr 10 or nr 30 blades to achieve a longer cut.
They cant be used on the longer blades.
Get the metal ones as they are stronger. The ones made of plastic are too flimsy and wont be able to go through the coat. Also, make sure it’s a snap-on comb -there is another type of attachment combs called slide on combs and they wont fit the regular blades. They are made for smaller trimmers and not clippers.
Attachment combs come in different lengths and again – they are numbered but the number doesn’t mean an actual length. But they always advertise the length in millimetres as well.