PlayOnline supports multiple handles for the same PlayOnline ID. They are your name on a friend list and in the chat rooms. You can have 16 characters per POL ID, 4 handles, and 8 Content IDs attached per handle. This was a concern when the platform was used to manage Content IDs of 6-7 different games (FFXI, Tetra Master, JongHoLow, Fantasy Earth, Front Mission Online, Dirge of Cerberus). You manage handles in the Navigator menu, right under Final Fantasy XI (previously Games). Link and Unlink can be used to switch characters between handles, since you can only have 8 of your characters there. Friend Lists are per handle, not per character.

If you have any Content IDs not linked to any handle, it will ask you to link them to the current one, which will only matter if you have more than 8 Content ID slots active. It can't link them if the handle is full, hence the error, and thus you need to make an additional handle for them to be able to link to. Otherwise you can say no to linking the unlinked Content IDs.