When fitting gear cables in the past I've always just used what I took to be a standard ferrule (a plastic or metal cyclinder with a hole for the cable) that slips over the end of the outer, on either end of the outer. It works fine.
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I've just ordered up an XTR gear cable kit for a new build and the write up on CRC describes inner and sealed outer caps and shows the familiar ferrule and a couple of other rubber caps, one with a long 'tube' on the end, so what looks like 3 types. I'm not quite sure what goes where and I was wondering if someone could enlighten me; I'll be running full length outers.
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Usually the one with the long bit on it goes nearest to the mech on the tab facing forwards along with one of the other rubber bits over it. Then the other ends are for normal routing.
You put the one with the long bit on it on the end of the cable as normal, then you fit it, and stick the rubber bit the other side of the stop covering the long bit. It's meant to be a kind of seal, although it does very easily get knocked out of place.
EDIT: when I order XTR cables the outer comes in bits so you won't be able to use full length outers anyway.