The Maidenhead locator system (which you correctly tagged) is described in detail in this Wikipedia page.
A short summary from that page is:
- Character pairs encode longitude first, and then latitude.
- The first pair (a field) encodes with base 18 and the letters "A" to "R".
- The second pair (square) encodes with base 10 and the digits "0" to "9".
- The third pair (subsquare) encodes with base 24 and the letters "a" to "x".
- The fourth pair (extended square) encodes with base 10 and the digits "0" to "9".
- The fifth and subsequent pairs are not formally defined, but recycling the third and fourth pair algorithms is one possible definition: BL11bh16oo66
On shortwave frequencies, positions are reported at square precision, and on VHF and UHF, subsquare precision is used. More precise position reports are very rarely used.
Note that this means the formal standard does not define anything past the first eight characters (four pairs), but that people tend to extend the system used in the third and fourth pair if they need to be more accurate.
Also note that this extension (only as far as five pairs, or ten characters) is the one used at the APRS website.