eLink protocols define a number of standard packets to transfer information. The contents and structure of these packets is tightly controlled in order to ensure maximum interoperability. These packets are often called APPT packets in reference to the underlying binary protocol name. Each packet has two standardised fields that define the contents of the packet.
APPT packets are defined by an application and each application is granted a unique F3 value. Within this space, that application is responsible for managing the individual packet types, identified by F2 values. For example, RetailMAX has been assigned the unique F3 value of "21", so whenever you see <f3>21</f3> you are able to instantly recognise this as a RetailMAX defined data structure.