the maintenance and feature streams do not depend on specific hardware. the difference is that the feature stream has additional, well, features
it was more expensive and shipped with one additional cd.
Was that pricing something specific to EU/Germany? At least in the US, the only question was whether or not you had a software support contract. If you had a support contract, you would receive a set of Overlay CDs containing both the f and m streams whenever there was a new release, and you were entitled to download the m or f installation images from the old SGI support site. If you didn't have a support contract, you could download a certain range of recent "m" images, ending in 6.5.22, but not the "f" images.
A minor note: The idea of different maintenance and feature streams was introduced in IRIX 6.5.2 and the streams were merged in 6.5.22. If you run a clean install of IRIX 6.5.22 or newer, the installer still prompts you to select a stream, but it makes no practical difference in the resulting installation, except for the output of uname and similar commands. Similarly, if you update a version of IRIX that was older than 6.5.22 to something newer than 6.5.22, it will retain the system's earlier "m" or "f" designation, even if it makes no practical difference.