Myfonts seems to make a difference between Melior in truetype, Melior Pro which is OpenType Postcript and Melior Com which is OpenType TrueType. It’s strange as Pro or Com would normally refer to language support and coverage, not to the file format.
I just looked at the different Melior fonts over at myfonts.com. What's the difference between the "Melior Com" and "Melior Pro" and "Melior"? At Linotype there seems to be only one, yet all claim to be from Linotype?
Bitstream got their start by making their own versions of the Linotype library.