PBX, also known as SPC exchange, completes telephone exchange between enterprises and public telecommunication network, and combines telephone, fax, modem and other functions. General term ext (extension) refers to the extension connected to the main line. The PBX is to handle the calls between extensions and then connect with the Public Switched Telephone Network (PSTN) through the trunk line.

For some telephone exchanges, a strictly defined classification cannot be unified. Due to the different names of manufacturers, customs, regions, product positioning and other factors. So you don't need to pursue it too deliberately, just purchase the appropriate equipment according to your own use requirements.