Ascii85 different from ASCII
Overview
The basic need for a binary-to-text encoding comes from a need to communicate arbitrary binary data over preexisting communications protocols that were designed to carry only English language human-readable text. Those communication protocols may only be 7-bit safe (and within that avoid certain ASCII control codes), and may require line breaks at certain maximum intervals, and may not maintain whitespace. Thus, only the 94 printable ASCII characters are "safe" to use to convey data.
Eighty-five is the minimum integral value of n such that n5 ≥ 2564; so any sequence of 4 bytes can be encoded as 5 symbols, as long as at least 85 distinct symbols are available. (Five radix-85 digits can represent the integers from 0 to 4 437 053 124 inclusive, which suffice to represent all 4 294 967 296 possible 4-byte sequences.)