The scope of PHI can be broad; however, medical information that an employer receives in its role as an employer (and not as the sponsor or administrator of a group health plan), is generally not PHI subject to HIPAA.
Alphabetic characters are the 26 letters of the alphabet. Numeric's are the ten digits. Alphanumerics are both. Some of the special characters on a standard keyboard do not work for usernames or passwords.
In a fixed-width binary code, each letter, digit, or other character is represented by a bit string of the same length; that bit string, interpreted as a binary number, is usually displayed in code tables in octal, decimal or hexadecimal notation. There are many character sets and many character encodings for them.