what is change control

It is a document that describes the additional
functionalities that are added after the
Business Requirement Document is signed off.
It can be updated in the old business 
requirement document or it can be a separate
document. (For example, in the Business
Requirement Document, on the login page,
there are User Name and Password fields.
The owner of the software wants to add, 
“If you do not have User Name and Password,
please click here.” This is a change. 
But this change came after the document 
is signed off by the Project Managers. 
Now this is a change control and comes
as a separate document. 
(It is also called Change Request, Modification Request).