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.