Seq stands for a-z or 0-9 which can be used only in Include and Exclude. When you write Include(Seq('a', 'm')) it means you are including a character from that sequence in a given position of your pattern. So if you have

Pattern(Include(Seq('a', 'm')), 'c')

That is sort of building the following set of strings.

ac bc cc . . .

When you use Exclude, it would be like you are picking a char that is not belonging to the sequence, like in:

Pattern(Exclude(Seq('0, 9')), c)

then you would end up with a set like:

ac bc cc . . .

It means, a set whose first character of the elements arent digits.