Alpha characters, more often called alphanumeric, are designed for computers and are comprised of the 26 alphabetic characters and the 10 Arabic numerals. Alpha characters also encompass letter variations for languages other than English.
The parameters of an alpha character is usually defined by what it is used for. For naming files, alpha characters are strictly limited to letters and numbers. However, in programming, symbols are normally permitted as alpha characters. This may be because various subsets of alpha characters were adopted to avoid misinterpretation by humans, depending on the application. An example of this is in aircraft seat maps. The letter "I" is skipped to avoid mistaking it as the number "1".