Alphanumeric

An alphanumeric code is one made only of letters and numbers.
However, humans can make mistakes, so the code is used so as to cut out common mistakes. Letters and numbers can be misread. The letters I, O and Q are similar to the numbers 1 and 0. 
"Merriam-Webster dictionary" suggests that the term "alphanumeric" may often additionally refer to other symbols, such as punctuation and mathematical symbols. This opens up a whole range of other possibilities, and a whole range of other confusions. It is not the usual way the term 'alphanumeric' is used. The "Shorter Oxford English Dictionary" defines the term as given above in our first line (though not quite in the same words).
Lastly, it is usual for alphanumeric codes to be "case insensitive", which means it does not matter whether you write them in upper case (capital letters) or lower case (small letters).