Hi Dip,
Functional view
The selection screen of a Standard Program (standard Tcode) is a Standard Selection screen.
By creating variants, you can customize the Standard selection screen say having 100 selection parameters to say 12 selection parameters incorporating Dynamic date display etc. This can be called as user defined Selection screen..
Technical view
The selection screen of any Z program can be called as a user-defined, but I'd like to call this as a default Selection screen of a Z program. Here too you can customize the Selection screen through variants.
Module-pool Programs
Above 2 cases are about Report programs. And the term Selection-screen mostly applies to Report programs.
Module pool programs are mulch-purpose, often used for Data-input s also. In such case the word Selection-screen does not apply.
Info set-Queries
Interestingly your question relates more to this area, where Logical Databases are used. Here the Infoset determines the Selection-parameters, which you can call as the Standard Selection-Screen. In addition to these parameters,if the developer adds few more selection-parameters, these appear in a separate block in the Selection-screen. These are user-defined Selection=parameters.
Jogeswara Rao K