What is the purpose of a 'tag' in a PLC?

A 'tag' in a Programmable Logic Controller (PLC) serves as an identifier for various elements within the control system, such as inputs, outputs, timers, counters, or other data types. The primary purpose of a tag is to store and represent the status of input and output modules, allowing the PLC to monitor and control the physical processes accurately.

When a tag is associated with input modules, it reflects the current state of those inputs (whether they are on or off), and similarly for output modules, it indicates whether the output is active or inactive. This allows programmers and operators to easily identify and manage the states of different components in the system, providing clear visibility into the operation of the PLC.

In the context of the other options, while assigning functions to outputs, indicating communication protocols, and backing up programming are crucial tasks within PLC systems, they do not encapsulate the core functional role of a 'tag.' Instead, these tasks revolve around the broader capabilities and configurations of the PLC, where tags specifically focus on the immediate status of inputs and outputs within the control logic.