The U:Line is a line in the cyclone configuration which informs the ircd which servers are "underworld" servers - that is, that bypass normal IRC limitations by manipulating certain features - usually used for IRC Services servers...

U:server name:*:*

In cyclone at least, there's also (in no particular order) a K:Line, Q:Line, C:Line, N:Line, X:Line, A:Line, M:Line, Y:Line, I:Line, O:Line, L:Line, H:Line and a P:Line. There's also a G:Line