the value returned is the value returned by DbCommand.ExecuteNonQuery. In general this is the value returned by the procedure on e.g. sql server, but it depends on the database type.
Calling a stored procedure should be done basically by adding a mapping to the procedure in the designer, generate code and then call the method. Manually constructing the StoredProcedureCall object is a possibility of course, but the code generator can do that for you.