A DODS server can accept a "constraint expression" contained in the URL query string. The DODS constraint expression describes how a DODS server should subsample a DODS dataset before sending the data back to the client. The details of the constraint expresssion syntax are covered in The OPeNDAP User Guide. What's important here is simply that the constraint expression is a logical expression with two clauses: projection and selection.
The "projection" clause of a constraint expression specifies the data variables requested by the client, and the "selection" clause specifies the condition under which the client wants them. That is, the projection clause might specify that the client wants to see oceanic temperature data, and the selection clause would specify that only records from below 1000 meters should be returned.