Sans-Titre Pro is a prototype font that grew extremely large over the years, so it was used mostly to collect characters and informations. The intention is to keep every information handy, stuffed in a single file. The font includes over 5800 glyphs and 30 OpenType Features, most of them for test and demonstration purposes only.
The focus is on content and functionality, and the role of the design is minimal, using the most basic lines, without too much personality or style.
The content is in constant development and includes unconventional approaches which means that the functions may not always follow standard procedures or the characters may not have a generic design (especially the ones in the Private Use Area and the new additions in general). Because of that, to prevent any confusion the font should not be used as a standard reference. For that you have the Unicode Standard and the Registered Features of OpenType by Microsoft/Adobe.