IPA: /nɒnˌælfəˈbɛtɪk/
KK: /nɑnˌæl fəˈbɛt ɪk/
Not related to or using the letters of the alphabet.
The code used in the game was nonalphabetic, consisting of symbols and numbers instead of letters.
Nonalphabetic → It is formed from "non-" (meaning not) and "alphabetic" (from Greek "alphabētos", meaning relating to the alphabet). The word "nonalphabetic" means not relating to or not using the alphabet.
Think of 'non-' meaning 'not' and 'alphabetic' relating to the alphabet — so nonalphabetic refers to something that does not use the alphabet.