[benjamin-lee]

This is because there's four bases and each can either be included or excluded from a given combination. So there are 4*2 = 16 combinations each of which with their own letter. In all honesty, these are pretty rarely used in practice these days except for N (any base) although they do sometimes show up when representing consensus sequences.

[pdimitar]

What do you mean that each base can be included or excluded? Isn't only one extra value needed? Sort of like nil?

[ac29]

Because there are notations for any combination of bases. There's a way to indicate "C or G", "A or T", "C, G, or A", etc.

[pdimitar]

Oh. So what's the grand total of all possible permutations of single and multiple (connected with an "or") values?

I'll also read through your links, thanks for posting them.