IBM Natural Question:

A control variable is used to dynamically set attribute and
colour definitions for a field or group of fields depending
upon a program condition. For example, Consider an order
processing screen that allows you to create new contracts,
modify existing contracts and look-up paid-up contracts.
The screen used for all these functions is the same.
However, when a paid-up contract is shown to the user, you
do not expect it to be displayed in a modifiable state. So,
you protect all the fields of the map and show it
like "read only". However, for modifying an existing, non-
paid-up map, you enable certain fields. So, control
variables can be used in scenarios like this to dynamically
set the attributes of the field.