Hello Chris,
You can use the "_cnd" parameter to set conditions/restrictions on the data being retrieved by the API.
The parameter is not documented on the official
http://kb.x-cart.com/ portal yet.
However, one of developers has created a small tutorial on using it.
The tutorial is in Russian, but Google Translate gives a somewhat correct translation:
http://dagdag-xcart5.blogspot.ru/2015/02/rest-api.html