Alphanumeric

An alphanumeric code is one made only of letters and numbers.

A Black-headed gull ringed with an alphanumeric plastic ring that makes it easier to read from a distance.

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.^[1]

1. In passenger aircraft seats are labelled by row number followed by column letter. For wide bodied jets, the seats can be 10 across, labeled ABC-DEFG-HJK. The letter I is skipped to avoid mistaking it as row number 1.
2. In the identification number used by motor vehicle manufacturers, the letters I, O and Q are left out. They are too similar to 1 or 0.
3. Tiny embossed letters are used to label pins on an V.35/M34 electrical connector. The letters I, O, Q, S and Z were dropped to ease eye strain with 1, 0, 5, 3,and 2. That is called the DEC alphabet after the company that first used it.
4. For alphanumerics that are frequently handwritten (in addition to I and O) V is avoided because it may look like U when written; and Z for its similarity to 2.

Merriam-Webster dictionary suggests that the term "alphanumeric" may often additionally refer to other symbols, such as punctuation and mathematical symbols.^[2] 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).

References

1. Slashed zero is a way of making the zero more distinct. Of course, that takes us beyond the basic alphanumeric typewriter symbols.
2. Merriam-Webster dictionary