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