The X:Line is a line in the ircd (possible only cyclone or dalnet ircd) which describes the passwords needed for the RESTART and DIE commands.

Format:
X:DIE password:RESTART password

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