PIPE's generally are REGISTERED shares - you can't sell unregistered shares. And PIPE are underwritten.
You are probrably thinking of a SECONDARY OFFERING which is open to the public a PIPE is not open and thats the difference. Now some pipes may be unregistred shares but not many PIPEs any more do unregistred except in extreme cases like they were doing with GS and C last year. Those were structured PIPEs and they did converts. Essentially a PIPE is a quick way for a small company to raise cash without the dog and pony shows.
I've been involved in many PIPEs so this will be my last comment on it. But MNTA was a PIPE, it wasn't open to the public PERIOD.