How much help??

How much help is okay for Correspondance chess? You are not allowed to use computers for analysing, but what about databases, opening books or endgame tables??

Quote from the terms of service:

"While a game is in progress you may not refer to chess engines, chess computers or be assisted by a third party. Endgame tablebases may not be consulted during play but you may reference books, databases consisting of previously played games between human players, and other pre-existing research materials."

Databases of human games are ok, books are ok, endgame tablebases no.