The first value is the precision and the second is the scale, so 18,0 is essentially 18 digits with 0 digits after the decimal place. If you had 18,2 for example, you would have 18 digits, two of which would come after the decimal…
example of 18,2: 1234567890123456.12
There is no functional difference between numeric and decimal, other that the name and I think I recall that numeric came first, as in an earlier version.
And to answer, “can I add (-10) in that column?” – Yes, you can.