From Halopedia, the Halo wiki
Unrelated to the UNSC Fleet Command, the Covenant FLEETCOM channel, part of the Covenant battle network, is intended for broadcasts across an entire fleet, rather than between a pair or more of ships. It seems to be the equivalent of the UNSC's FLEETCOM 7. It is similar to the INTERSHIPCOM, albeit on a much grander scale.
It should be noted that because of its scale and ability to broadcast over the entire Covenant fleet, it most likely has a restricted use only to San'Shyuum and very high-ranking Sangheili.