Accessories are items that are required to be tied to a “host” string item to define a system. This is done to be able to logically represent string items which are too complex to be given as just a stand-alone item of the string. Only two such “host” string items, or string items with accessories, have been defined to date. These are the electrical submersible pump (ESP) and downhole permanent gauge (DHPG) systems.
Source: API STD 689, Collection and Exchange of Reliability and Maintenance Data for Equipment, First Edition, July 2007. Global Standards