Please start by looking at the current Talk Group Names on the RadioReference Site. Many changes were made to Talk Group names and descriptions earlier this year, including the Talk Groups formerly identified as Emergency Response Teams (ERT).
37488, 37520, 27552 and 37584 are used by ValleyCom’s Police Agencies and are currently designated as Special Operations (SpecOps). As dpm3 notes (09/28/2014 @ 7:49PM), the Talk Groups are used for special operations.
As to which agencies may be heard on the ValleyCom SpecOps and ValleyCom NARC Talk Groups, they are principally used by ValleyCom’s police agencies. However, other police agencies may use them. With that said, agencies are becoming increasingly aware of the value of allowing others to access Talk Groups. Radios have improved and have the capacity to allow many more Talk Groups. The best illustration of this is King County Sheriff Aviation and Marine Operations. Radios used by these specialized resources may be configured with many Talk Groups to allow the Aviation and Marine resources to communicate with units involved in an incident.
Use of Tactical (TAC) Talk Groups varies among agencies. Generally, TACs are used for special events or more routinely when a high priority event is using the primary Talk Group. When the primary Talk Group is being used for a high priority incident other units (and calls) will switch to TAC so as to not interfere with the high priority incident on the primary Talk Group.
Public Safety Operations (PSOPS) Talk Groups are used to provide (typically) law enforcement units from multiple agencies with access to a dispatcher and other units involved in a (usually) quickly evolving incident. In most cases, a PSOPS Talk Group is patched to another Talk Group. Here’s an illustration: Report of shots fired in Northwest Renton, with the vehicle travelling northwest on SW Sunset Boulevard. The ValleyCom Renton Police Dispatcher will Patch Renton Primary to PSOPS S1. King County Sheriff and Seattle Police Communications Centers will be notified of the vehicle description, direction of travel and advised to switch units to PSOPS S1. The reason for this protocol is so that the Renton units can concentrate on finding / pursuing the vehicle and don’t need to change Talk Groups. KCSO and SPD units are not involved in the pursuit and have some time to change Talk Groups, hear the direction of travel and support the event.
PSOPS Talk Groups used in King County are divided at I90. S1 and S2 are used south of I90. N1 and N2 are used north of I90. With that said, all radios in King County should have all four PSOPS Talk Group. If both of the South or North PSOPS Talk Groups are used, a dispatcher may patch any of the PSOPS Talk Groups for use anywhere in King County.
If any of this is confusing, let me know and I will make another run at it.