Purpose of "RelationForums" in the Object Cache

[Adam Hun]

I'm wrapping my head around how JForum works and I found something called relationForums.

I don't know what it exactly does or what it is needed for.

I took a snapshot of DefaultCacheEngine. Well, simply .toString() and printed it.

So it looked something like this among other things.

...
forumRepository={
		loaded=1, 
		relationForums={3=1, 10=1, 4=1, 9=5, 8=4, 13=5, 11=4, 12=4}, 
		lastUser=net.jforum.entities.User@717a23b3, 
		1=[****, id=1, order=4, /jforum/forums/show/3.do], 
		5=[****, id=5, order=1, /jforum/forums/show/9.do], 
		4=[****, id=4, order=5, /jforum/forums/show/8.do], 
		categoriesSet=[
		[****, id=5, order=1, /jforum/forums/show/9.do], 
		[****, id=1, order=4, /jforum/forums/show/3.do], 
		[****, id=4, order=5, /jforum/forums/show/8.do]], 
		mostUsersEverOnline=net.jforum.entities.MostUsersEverOnline@63e0521c, 
		totalMessages=0, 
		totalUsers=13}}

What do those pairs of numbers mean ?
What are those for ?