It applies to digital local and combined exchanges for telephony in Integrated Digital Networks (IDN) and to local and combined exchanges in an Integrated Services Digital Network (ISDN), and also to mixed (analoguddigital) networks.
ITU-T Q.512-1995 history
1995ITU-T Q.512-1995 Digital Exchange Interfaces for Subscriber Access - Digital Exchanges (Study Group 11) 14 pp