What is MST?

MST is short for "Monetary Standard".

Or also:
MST = Master = Currency of the Company
Cur = Currency (needs also a transdate or exchangeratefield)

AmountMST refers to the base currency you keep your records of the system in, i.e. "Company currency".
So for example when recording an invoice you keep the amounts both in your company currency, i.e. MST (for example DKK), and the invoice currency, for example EUR.

source: http://blogs.msdn.com/b/palle_agermark/archive/2007/04/24/what-is-mst.aspx