Each line device is assigned one or more addresses. An address corresponds to a telephone directory number, and it is actually assigned twice: First, by the telephone company at the switch, and second, by the user while configuring the local system. If a telephone number is changed at the switch, the user will normally need to assign the new number at the local system, although some systems can be sophisticated enough to perform the reassignment without human control.
After addresses have been assigned to lines, TAPI assigns address identifiers to addresses. An address identifier is a number between 0 and the number of addresses on the line minus one. Because each address depends on its line to exist, the address's identifier is meaningful only in the context of the associated line device. For this reason, an address name consists of not only the address identifier, but also an identifier of the line. It serves as a kind of shorthand, an easy way for programmers to identify addresses.