Altar Room

"Attic? You mean the altar room, don't you? The rules are very clear: every witch must keep their Book of Shadows in a sacred and protected spot."

- Natalie mentioning them to the Charmed Ones.

An altar room is where a witch keeps and uses their Book of Shadows. The Council of Elders wrote a rule that all witches keep their books in that sacred room, while whitelighters enforce that rule. However, Leo Wyatt neglected to tell that to the Charmed Ones, who kept theirs in their attic.