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. 

I hope this answers your query.

A control variable is is defined with format C and can be
used to assign field attributes dynamically. It could be
assigned during the map design (element level) as well as
during the programming (statement level).

Example : 
DEFINE DATA LOCAL
1 #ATT(C)
1 #A
END-DEFINE
MOVE (AD=I CD=BL) TO #ATT
INPUT #A (CV=#ATT)

In a layman terms, a control variable (#CV (C) does only 
two but very useful functions.

1. Do you want the user to overlay the field with new value 
or you want to protect the field, for example, if an Order# 
is a system generated number, you would want that to be 
modified

2. If the fields not protected, was it a modified?
For example, if the SSN was modified then validate it 
otherwise take no actions against the field. Juts my 2 cents