MS SQL Server Concepts and Programming Question:

Answer #1
If an output parameter is defined in a stored procedure, the execution statement must provide a variable to receive the output value in the format: "@variable_name OUTPUT" or "@parameter_name = @variable_name OUTPUT". The following tutorial exercise gives you a good example:

-- Using @variable format
DECLARE @message VARCHAR(40);
EXECUTE diff_in_days
'01-Jan-2007',
'11-May-2007',
@message OUTPUT;
PRINT @message;
GO
May 11, 2007 - Jan 01, 2007 = 130

-- Using @parameter = @variable format
DECLARE @message VARCHAR(40);
EXEC diff_in_days
@start_date='01-Jan-2007',
@end_date='11-May-2007',
@days = @message OUTPUT;
PRINT @message;
GO
May 11, 2007 - Jan 01, 2007 = 130

Answer #2
-- Mixed formats are not allowed
DECLARE @message VARCHAR(40);
EXEC diff_in_days
@start_date='01-Jan-2007',
@end_date='11-May-2007',
@message OUTPUT;
PRINT @message;
GO
Msg 119, Level 15, State 1, Line 2
Must pass parameter number 2 and subsequent parameters
as '@name = value'. After the form '@name = value' has
been used, all subsequent parameters must be passed in
the form '@name = value'.