The difference is that the microprogram is usually only developed by the processor manufacturer and works intimately with the computer hardware.
Another means of employing microassembler-generated microprograms is in allowing the same hardware to run different instruction sets.
For example, through the use of macro-assembler-like capabilities, Digital Equipment Corporation used their MICRO2 microassembler for a very wide range of computer architectures and implementations.
In the process of microcode assembly it is helpful to verify the microprogram with emulation tools before distribution.
Nowadays, microcoding has experienced a revival, since it is possible to correct and optimize the firmware of processing units already manufactured or sold, in order to adapt to specific operating systems or to fix hardware bugs.