Literary / historical register. Modern speech uses 仆人 (servant) or 助手 (assistant). 仆从 is preferred in historical fiction, court scenes, and political writing, especially the phrase 仆从国 (puppet state, vassal state).
history
In WWII Chinese sources, Nazi-allied states (Romania, Hungary, Bulgaria etc.) are routinely called 仆从国, the 'satellite states' of the Axis.
