Since you do not specify a size for that given element it is assumed to be 1 byte. So the variable a does hold a bitstring of length 8 or a binary of length 1, and should be even a string (while not printable, it does only contain valid codepoints)
Remember the picture from above, every string is a binary and every binary is a bitstring, but not necessarily the other way round.
A bitstring is a binary if and only if it has a number of bits that is evenly devisiable by 8.
A binary is a string if and only if it does only contain valid unicode codepoints encoded in UTF-8.
So as you can see binary and string are true subsets of binary and string is a true subset of bitstring and binary.
