A_MSSQL_DEFAULT_CONNECTION specifies the name of the server to which the run-time system will connect. This variable is checked only if the DSQUERY environment variable has not been set. If neither DSQUERY nor A_MSSQL_DEFAULT_CONNECTION is set, the server is named localhost. To reference tables in another server, open the file servername.database.owner.table .
Example
Suppose you have two servers, one named TOM and one named HARRY. If most of the tables you want to access are on the server HARRY, you should set:
A_MSSQL_DEFAULT_CONNECTION HARRY
For those occasions when you want to access the TOM server, you could open the file this way:
TOM.stores.johndoe.purch1