There are over 500 individual devices supported by the HART protocol. Each of those devices has it's own set of "device specific" commands. These are above and beyond the "universal" commands which are common to all HART devices, and the "standard practices" commands that are common to most HART devices.
To really understand what HART can and cannot do within a particular device, you really have to go back to the device manufacturer's documentation/manual.
The actual interface to HART devices can be accomplished with PC-based software or utilizing a HART communicator. No matter which manufacturer's communicator you use, you must have the device specific files, "DD's" (device descriptor) or DOF (device object file), in order to read or change any the of the "standard practices" or "device specific" parameters.