Room packs are byte values assigned to each room in the overworld.
ZOLE supports modifying room packs as of v4.6.01.
Room packs are primarily used in Oracle of Seasons to divide rooms based on their seasons. Crossing between two rooms of different packs will trigger a fadeout transition and cause the season to be rechecked.
A value of $ff is used in the animal companion region. It seems to tell the game to load a different room based on which animal you have.
Room packs are mostly unused in Ages (everything is in pack $01). However, the Maku Tree in Ages has a value of $80 for its room pack. It seems that, in Ages, the value must be $80 or above in order for fadeout transitions to occur, but this is not the case in Seasons.
The animal companion region has value $7f for its room pack. Like Seasons, this tells it to change which room to load based on your companion.
Ages has distinct room pack values for the past and the present, while Seasons shares the same data for all 4 seasons (for obvious reasons).