Occultist implements "domain"?

[IconoclasticScream]

I admit to not being well-versed in the Occult Adventures classes yet, so if the answer to this is painfully obvious my apologies in advance. When I add the implements to an occultist, the name of the implement appears in red followed by "(????)" and an empty text box. The validation report says that a "Domain must be specified," but for the life of me I cannot figure out what that means or what's supposed to be in that text box.