When you go to this site, you may notice the Bentley logo there, disregard it if you think it's DGN based.
NO.
Rebis is DWG based.
Bentley for some reason decided to buy them out.
Bentley is MicroStation (DGN). ... wierd.
I do believe Rebis does this "BUT I CAN'T CONFIRM".
I've read in magazines that the software is very powerful and "I BELIEVE" THIS was a "MAJOR" factor for Bentley to buy out the company and rights to the software.
Logically why would a competitor of AutoDesk "BUY" a DWG based software? Especially, when ALL Bentley products are DGN based.
Obviously Bentley is finally seeing that AutoCAD files are every where and they better get in before it's really too late.
ANYWAY ...
It's all speculation.
I thought you may want to look into it since it did seem valid.
I'm typing this slow so its a whisper, okay. It is my opinion that the only reason MSTA is still around is they hacked open the DWG format so that MSTA could read ACAD DWG's. But that just me.