Bryce919er's description is very accurate.
For sure, the old Serial Port Power Commanders are fine and dandy and still valid for use - especially if you have a tight budget.
Unless you have an old computer with a Serial Port connector, you'll need a Serial Port X USB adaptor device if you want to be able to change maps.
There is a decent library of .map format maps.
Any .djm map you want to try but must be in .map format for use in the Serial Port Power Commander, is easily manually created with the Serial Port era Power Commander software.
I've done such work, it's super easy, and it worked like a charm.
(Conversely, any .map format map can be auto converted to .djm format by the PCIIIusb era software.)