Every PFB file contains an "encoding vector"; this is a plain text line embedded near the head of the PFB file. Encoding vectors are of two types: AdobeStandardEncoding and everything else. Adobe usually uses the label "FontSpecific" for fonts that are not encoded according to AdobeStandardEncoding, and I use it as a cover term here for any such font.
If you look at the readable plain text information at the head of a FontSpecific type font, it includes a range of text that begins:
/Encoding 256 array
followed by a bunch of lines, each of which includes a number (which corresponds to a cell in the font layout) and the name of the glyph that lives in that cell. The unreadable binary data below this array specification lists the name of each glyph and the PostScript instructions for how the glyph is to be drawn. There may be PostScript code for drawing glyphs that are not included in the mapping array, but only glyphs mentioned in the array specification are available to applications.
FontSpecific type fonts are comparable to the bitmapped fonts I used under DOS. Each character physically is assigned to a specific cell within the font file and operating environments are not allowed to remap these. The glyph in cell 246 will be the same in both Windows and OS/2.