Add-On Instructions play a critical role in PLC programming. They give a programmer the ability to define custom instructions in order to maximize code reusability and simplify development. An Add-On Instruction or AOI is basically a function which is defined by the programmer and works in the same manner as the one provided by Rockwell.
This is a companion discussion topic for the original entry at https://www.solisplc.com/tutorials/add-on-instructions-programming-aoi-rslogix-studio-5000-plc-programming-tutorial-example-logic