If you wish to use a cassette/hub with the same number of sprocket (or fewer) with a derailer/shifter of a different system, you may need to replace some of the spacers to change the spacing of the cassette. Modern indexing systems are so forgiving, however, that many theoretically incompatible combinations actually work fairly well.

In particular, for reasons that are not quite clear, 9-speed hubs/cassettes seem to work pretty well with the opposite 9-speed derailer/shifter brand.