In POTS, exactly one channel exists on a line, and this is used exclusively for voice. With ISDN, at least three (and as many as 31) channels can exist on a line concurrently. The simplest form of Win32 Telephony functionality involves POTS applications that handle a single line using its single channel. 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 not at the same time.
In the simplest case, one line has exactly one address (telephone number). In cases where lines carry two or more channels, each channel usually has its own address, which means that a line has as many addresses as it has channels. To make addresses easier to manipulate, TAPI assigns address identifiers to them, in a contiguous range of integers starting with 0. This lets the software interfaces refer to addresses independently of the address syntax on any particular telephone network.