FUD means fully Undetectable or no anti virus detect. Fud crypters can be used to Encrypt Viruses, Rat, keyloggers, some spywares tools. Exact means hide virus from av. cypter simply assign hidden values to each individual code within source code. thus, the source code becomes hidden. so your victim can not understand that its virus(fud).
Anti virus work on signatures base. Anti virus has virus definitions, signatures stored in its database. when scaning a file, anti-virus searches for these virus definitions or signatures in a file and if its finds these signatures in file, anti virus alert file as virus otherwise no virus.
If you are trying to crypt virus using publicaly known available crypter. They are detected by anti virus. crypter remain FUD for 1 week after, their publc relese.
you dont use free crypter because they are public relese. you can use licence versions crypter or build your own crypter.