Internal payloads – common to most 1U CubeSats. Constrained by internal volumes, walls, connector pinouts.
Semi-internal payloads – e.g. GeneSat, PharmaSat where the payload is an integral but separable part of the overall CubeSat structure. May have more internal volume available, usually employ a custom electrical interface, are tied to the structural design of the CubeSat structure
External payloads – “bolt-on”, like 4th-generation CSK. Have the potential to be highly modular, since tied primarily to CubeSat specification for dimensional issues, not to CubeSat itself.