Major accounts or key accounts usually refers to a company's most important clients. The "major" refers to how important the customer is to the company, not to how big the customer is. I also think National means "domestic", versus international. It doesn't matter if they have branches nationwide (all over the country) or not.

MGA - Michael Gregg Agency; About Us
... client relationships. We have served three important clients for nearly 10 years and several other major accounts for over 5 years. ...
www.donet.com/~mgregg/docs/aboutus.html - 23k - Cached - Similar pages

Key Personnel in Government Research - Decision Analyst
... Mr. Riggins has managed dozens of major accounts at Decision Analyst. ... analyses or consulting services for a wide range of important clients at Decision Analyst ...
www.decisionanalyst.com/Services/KeyPersonnel.asp