Why ASCII letter A has a Hamming Distance of 3? My question is what does [this construction gives a Hamming Distance a distance of 3] means? Why the Hamming Distance of ASCII letter A is 3? How do you determine the Hamming Distance is 3?

This is an excerpt from Computer Network by Andrew S. Tanenbaum page 206.
Tanenbaum is not saying that ASCII character "A" has a particular Hamming distance. Computerphile has a video that may clear things up.