Occasionally proprietary does not mean that it is secret,
but just
non-standard, and specific to a particular
vendor.
Some proprietary things are still
open--with full specifications published. In these cases, they are used
either because the
standard didn't do what the
designer wanted, so proprietary may actually be better,
or the proprietary item is selected to lock you into
using their connectors or mounting brackets or whatever.
Sun has a habit of designing
proprietary connectors and then allowing (encouraging) them to become industry standards.
Or, it could just be a proprietary extension to an existing
standard.