In POTS, exactly one channel exists on a line, and is used exclusively for voice. With ISDN, at least three (and as many as 30 or more) channels can exist on a line simultaneously. Currently, most TAPI functionality involves POTS applications that handle a single line using its single channel because ISDN hardware is not yet widespread. In POTS, an application that wants to transmit data communicates over one line, and a voice application communicates over another line—both of these applications could use the same line, but if so, not at the same time.
In general, one line has exactly one address (telephone number). In cases where lines carry two or more channels, each channel can have its own address, which means that a line has as many addresses as it has channels. TAPI assigns address identifiers to these different addresses to make it easier to manipulate them.