These are roughly comparable to IP addresses used in the Internet Protocol; they can specify a piece of equipment connected to an Asynchronous Transfer Mode (ATM) network.
A specific stream, analogous to a TCP/IP port or socket, is specified by using a transport service access point (TSAP).
ATM can also use a presentation (PSAP) and session (SSAP) access point, but these may also be unspecified; this is up to the application.
One of the schemes to generate NSAPs uses E.164 which is the addressing format describing telephone numbers.
They are usually assigned by the network management/network operations centre personnel and agreed upon within an organization to be unique (to that organization and based on geographical location using country code telephone prefixes) and are required before any operational connectivity is established at the commissioning stage.