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I'm wondering how "haben" and "sein" are used in the future perfect tense.
Say
Ich werde gegessen haben
means "I will have eaten" and it uses "haben".
Then
Wann werden sie gegangen sein?
can mean "When will they have gone?" but it uses "sein".
Are the different usages fixed combinations (i.e. needs to memorize them all), or it's associated with certain type of words / actions (i.e. similar to "where" vs. "go to where")?
Just like in Perfekt (present perfect) and Plusquamperfekt (past perfect), Futur II (future perfect) uses either "haben" or "sein", and in some cases can use both. There's no difference in the choice of the auxiliary verb between Futur II and the other two tenses.
In general, many verbs use "haben". Verbs of movement use "sein", as do verbs expressing a resultant state or state change. When verbs can use both, "sein" expresses again a resultant state, while "haben" expresses an action.
Details for example at canoo.
Future perfect is "future" combined with "perfect". So whichever helper (haben, sein) you'd use for perfect, that'll be the one for "future perfect", too. If you don't know which one to use for "perfect" then "future perfect" is not something you should be worrying about.
