ax, bx, cx, dx are 16 bit registers referring to the lower 16 bits of rax, rbx, rcx, and rdx respectively. Bits 0..8 can be referred as al/bl/cl/dl, bits 8..16 as ah/bh/ch/dh.
ax, bx, cx, dx are 16 bit registers referring to the lower 16 bits of rax, rbx, rcx, and rdx respectively. Bits 0..8 can be referred as al/bl/cl/dl, bits 8..16 as ah/bh/ch/dh.
It's ambiguous, but I believe the comment you are replying to suggests that the sentence should read:
>> Additionally, the higher 8 bits of ax, bx, cx and dx can be referred to as ah, bh, ch and dh.