Project 25 (P25 or APCO-25) is a suite of standards for interoperable digital two-way radio
products. P25 was developed by public safety professionals in North America and has gained
acceptance for public safety, security, public service, and commercial applications worldwide.
P25 radios are a direct replacement for analog UHF (typically FM) radios, adding the ability
to transfer data as well as voice for more natural implementations of encryption and text
messaging. P25 radios are commonly implemented by dispatch organizations, such as police,
fire, ambulance and emergency rescue service, using vehicle-mounted radios combined with
repeaters and handheld walkie-talkie use.
Source:https://en.wikipedia.org/wiki/Project_25
P25 repeaters use whats called a NAC (Network Access Code) in place of the PL tone/CTCSS access
common to amateur repeaters. This a 12-bit prefix that is attached to every packet (including
the digitized voice packets). 12 bits means 4096 possible access codes, quite a lot more room
than the previous analog methods. There are a few special codes, a default and an "open" code
that will pass all traffic.
Source:
https://ham.stackexchange.com/questions/973/how-to-access-a-p25-repeater#975
P25 can be used over a network (or refectors) by employing a MMDVM configured to a default Talk Group, say 10700 and a NAC almost always set to 293 . To change to another Talk Group it is a simple matter of transmitting on the newly selected Talk Group and wait a second or two and yiuwill now be on that Talk Group.