AFI (Authority and Format Identifier) specifies the format of the IDI , the authority responsible for assigning values of NSAP addresses to different system.
IDI (Initial Domain Identifier) specifies the network addressing domain from which the format of DSP is allocated.
DSP (Domain specific part) typically is a combination of administrative and topological information. i.e. area , organisation, subnetwork, hostID,....
SEL (Selector) is used to select among multiple transport layer entities in a system.