All classes, Servants, and similar data not yet accessible by just playing the game (but uncovered through data-mining, Chinese telepathy devices, &c.) are considered spoilers and should be tagged throughout the forum. Officially released trailers and the like count as being accessible for the game, so you can talk about the fact that Arjuna exists, but not about yet-to-come plot events concerning him that have been uncovered by extracting files.
The GO story thread is fair game for all story spoilers available so far, but not for spoilers which aren't actually in the game yet.
Forumwide, and especially in GO General, you are expected to exercise reasonable discretion as to your use of story spoilers concerning events which are accessible in-game but which a considerable portion of the user-base may not yet have reached. If you know something because you've been spending five thousand yen per day and have blitzed through the latest release in six hours, consider that not everyone is up to your standards. If you do not understand what qualifies as reasonable discretion, just save everyone a headache and tag it.
(Policy is subject to reasonable revision if any gaping holes are uncovered).