I am not aware about other technologies such as GSM, WCDMA but in LTE technology, APN stands for access point name. A subscriber may have subscribed for the different services offered by its service provider. Operator segregates their services and group them through APN. For example, when LTE user wants to access internet browsing, it makes connection with internet APN and when it wants to make VoLTE call, it makes connection with different APN. These different APN names are used by network to select different packet data network for different kind of services.