Can I use DQL to calculate the value of a Custom Field?

The documentation for Custom Fields states that:

Custom fields allow you to extend your types with custom logic as well as make joins between your local data and remote data.

Am I right to assume that custom logic refers to @custom(dql:""" query{} """), because the example code on the page only shows @custom(http:"""? Can I use DQL to fetch a value for a custom field?