Back when I was the Raid Coordinator in Epic Fail, one of, if not the largest guild on Orien at the time (well before airships were introduces), I would put up LFMs like "EF Shroud" or "EF VoD". It just made it much easier to get the group together for raids, especially for raids like DQ where you have to split into two teams for the preraid. At the time EF was one of the few guilds that raided on Orien, but by no means was it a "raiding guild" as the guild was large and heterogeneous and raiding was certainly not the top priority for most of the members.
However, several raiding guilds that do prioritize raiding have emerged over the years. Interestingly, most of the leaders and original members of these raiding guilds were once members of EF. OR, Myths, HB, PK, and others I'm surely forgetting.
In any case, if you see an LFM that simply has a guild acronym, especially if there is no quest/raid specified, it's likely a closed guild/channel quest or raid.
I thought this was commonly understood, but for whatever reason, I'm getting 20-30 tells a night asking "What is OR?" "What are you running?"
This will probably fall on deaf ears and will likely not even reach the population that questions. I don't know why there's this sudden explosion of inquiring minds, but I have two hypotheses:
(1) There hasn't been as many of these guild acronym LFMs recently as many raiding guilds have slowed or gone on a raiding hiatus in order to prepare for the XPAC (TR'ing seemed like a better use of time than grinding gear that would be obsolete).
(2) Mass stoning? Sudden burst of 16-20s that are new to raiding and epics?
Also, a bit of a tangent but:
(1) If you are invited to a group to "pike a completion" but told you won't be able to loot chests b/c of a lockout (which most raids have)... This is a courtesy, a favor to you. You'll collect the end reward and be one completion close to a 20nth list. If you respond rudely, expect to be /squelch'd.
(2) If you really want to insinuate yourself into a closed LFM, don't pepper the leader and non-anonymous members of the guild you find on the /who list with 20+ tells about how much you need the completion/token/rewards. That's just bad salesmanship. If you want in, you should be able to convince the gatekeeper (and no one else) of what you can contribute - not what you need.
(3) If you see a closed, guild LFM in which everyone in the group is already in the quest, that's a solid indication that they have already started the quest. The fact that the LFM is still up is not an invitation for you. It's there so any guildie/channelmate who sees it can pop in for a free (chestless) completion, or it's still up for convenience so the group can quickly jump from raid to raid as a few members will invariably have to relog to alts.