When changing the category for an incident, in activity log is listed something like: 
FIELD='Request Area' OLD='AAA' NEW='BBB' 
where "Request Area" field does not exists in an incident detail page. 

Can this message be changed?

CA Service Desk Manager 14.1
CA Service Desk Manager 17.1

You can influence the activity log text by a mod file in site/mods/majic 

MODIFY cr category DISPLAY_NAME "Category"; 

Nevertheless, you might have a mismatch between displayed GUI field label and the generated activity log text. 

As you know the cr object is in use for Requests, Incidents and Problems. The category attribute is labeled "Request Area", "Incident Area" or "Problem Area" and therefore will not match to a single DISPLAY_NAME anyway. Unless you would change the labels in the GUI to a unique single label for all ticket types.