"u" & "o" next to ECN on mkt depth screen

[n23mc]

What does the "o" & "u" represent next to the ECN's on the market depth screen. They're displayed to the left of the ecn name in the MMID column.

[JayF]

The four-letter ids are capitalized for NASDAQ market participants and are in lower case for non-participants. Non participants are in lower case letters and come in 2 types. The "u" in front of some of them is for UTP (Unlisted Trading Participants) and applies to ones like cinn, arcx, & chxe. The "o" in front of some of them is for OMDF (OTC Montage Data Feed) and applies for ones like inca and ntrd.