first parameter x of the decimal (x, y)

As I can determine the first parameter of the decimal data type, sometimes I had to put 10 and sometimes I put the value of 11, I did a lot of tests. There is some function that allows me to determine the value of the first parameter x of the decimal (x, y)

select PERIODO, SUM(CAST(DEBE AS decimal(10,2))) AS DEBE_MENSUAL,
            SUM(CAST(HABER AS decimal(10,2))) AS HABER_MENSUAL
FROM [dbo].[Octubre]
WHERE ISNUMERIC(DEBE)=1 AND ISNUMERIC(HABER) = 1
GROUP BY PERIODO


UNION ALL
select PERIODO, SUM(CAST(DEBE AS decimal(11,2))) AS DEBE_MENSUAL
            , SUM(CAST(HABER AS decimal(11,2))) AS HABER_MENSUAL
FROM [dbo].[Diciembre]
WHERE ISNUMERIC(DEBE)=1 AND ISNUMERIC(HABER) = 1
GROUP BY PERIODO