I think - this may not be right - Open mode is like Uniden's ID Search mode -- you hear every talkgroup, regardless. Closed mode is like ID Scan - you only hear the talkgroups for which you've made a scan list
The ability to open and close banks allows you to listen to only the talk groups you programmed into that bank (closed mode) to all the radio traffic on the trunking system you are monitoring (open mode). I programmed all the fire, ems, air ops talk groups into the bank 1 of my PRO 163 and set the bank to closed. I did the same for bank 2 but with the police talk groups. The - icon indicates closed mode while + indicates open.