The wide adoption of the Z39.50 protocol from the Libraries exposes their abilities to participate in a distributed environment. In spite of the protocol specification of a unified global access mechanism, query failures and/or inconsistent answers are the pending issues when searching many sources due to the variant or poor implementations. The elimination of these issues heavily depends on the ability of the client to make decisions prior to initiating search requests, utilizing the knowledge of the supported search capabilities of each source. To effectively reformulate such requests, we propose a Datalog based description for capturing the knowledge about the supported search capabilities of a Z39.50 source. We assume that the accessible sources can answer some but possibly not all queries over their data, and we describe a model for their supported search capabilities using a set of parameterized queries, according to the Relational Query Description Language (RQDL) specification.