A compact PROFIBUS DP slave has an I/O configuration that is frozen at manufacturing time (e.g. a 32-bit encoder, a 4-module pressure transmitter), which simplifies configuration but limits flexibility.

A compact slave is the "all-in-one" pattern: a single configuration is declared in the GSD, the engineer has no module choices to make in the engineering tool, and the trade-off is the absence of any future I/O extension. Typical examples are encoders, process transmitters and basic drives where the device function is fixed by design.

For a compact slave, do not waste time looking for "module selection" in the engineering tool: there is nothing to choose, the configuration is what the GSD declares.