Let me show my ignorance by replying (I trade NYSE almost exclusively): SelectNet gives the market maker the right to post SelectNet bid and asks that they don't have to honor, ie they can back away even after you preference them on SelectNet. This is called "non-liability ". This is a particularly nasty problem, since the MM has 30 seconds to make that decision and can hold your order during that time.
After the introduction of SuperSOES it's my understanding that all SelectNet order are "non-liable", ie SuperSOES is the only trading avenue where MM bid/ask are commitments.
Others please chime in and make corrections!