There are raids that only let me kill their bosses once, no matter their difficulty (Icecrown). And there are raids that use a separate lockout for each difficulty (Firelands). I can’t seem to find any coherent rule that works in telling me in advance if I will be able to run a raid for loot multiple times. Is there one?
The latter started in Cataclysm. ICC was more flexible than it’s predecessors. It allowed you to skip ahead in your lockout. Everything before ICC is an old style lockout. Everything after ICC has separate lockouts for multiple difficulties. Flexible lockouts started with Siege of Orgrimmar at the end of MoP.
Mogu’Shan Vaults didn’t let me enter in others difficulties once I completed it in HM25
I have been skipping around thru various raids (and dungeons) to complete questlines, earn achievements and for mounts, mogs, mats, recipes, ect and I have yet to find any rhyme or reason.
Also everything is subject to change without notice, let alone rhyme or reason. When I started doing the staff questline in Firelands, I was able to do 25 mans for achievements at the same time and, then, one week… I couldn’t.
(And before any explains it is because they turned it into flex mode instead of just adding flex mode, I know. Still no rhyme or reason to it.)
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I do wish they would retroactively enforce the current separate difficulty lockouts that modern raids have. It’s terribly confusing as of now.
It would be better in my opinion if we just got one lockout per raid, with the highest difficulty rewarding loot from all other ones. So if a raid’s highest difficulty was Mythic, it would also reward loot from LFR, normal, heroic… Only for legacy content of course.
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They changed the lockout rules again at the beginning of Shadowlands. They killed the Lich King skip strat 
I…thought only SoO and latter raids allowed you to run multiple difficulties on one lock out. 