Why can't D store all UTF-8 code units in char type? (not really understanding explanation)

[thebluepandabear]

>
> That's not a utf-8 code unit.

Hm, that specifically might not be. The thing is, I thought a 
UTF-8 code unit can store 1-4 bytes for each character, so how is 
it right to say that `char` is a utf-8 code unit, it seems like 
it's just an ASCII code unit.