This really isn't an Excel question but there are sharp people out
there that might know what I'm looking for.

For a POSITIVE number (like +2, +68, +106), the value of the rightmost
digit is the rightmost digit. For example:

1 = +1
2 = +2
3 = +3
etc

But, for a NEGATIVE number (examples, -1, -17, -543), the value of the
rightmost digit is an alphabetic- or special-character. Something
like:

A = -1
B = -2
C = -3
etc.

What term describes this numbering-scheme?