Board Thread:Support Requests - Getting Technical/@comment-7630509-20151101220323/@comment-15636815-20151112222856

"Necessary" is a relative term. Mostly, it depends on how clean you want to keep your category structure. If you use template documentation (and everyone should), then without the includeonly tags, the template itself will be in the same category as all the article pages the template puts there. If B is the only template on your entire wiki that puts articles into categories, then it's probably no big deal. Except that the chance of it remaining the only one is essentially zero. If it's one of 10, or 30, or 200 templates that do the same thing, then your category tree will be more cluttered than a five-year-old's bedroom the day after Christmas. Since the #1 rule at Wikia every single day is unpredictability, I recommend creating the habit early and never letting go of it.

TL;DR - Yes, the includeonly tags are necessary around links that populate categories, when used inside templates.

The code in A should look something like this: Short, sweet, simple, to the point, easy to remember. Basically, just add  to the end of whatever you have in A that tells B to do whatever it tells it to do. Then add the just-as-simple code to B that we discussed earlier, and you're done! :-D