FSX Account Setup

I would guess that it means it must include both letters and numbers.
Alpha: Alphabet, comes from greek letters (alpha, beta, gamma, etc.)
Numeric: To do with numbers.
If I am wrong which is quite likely, and you do find the correct definition, please post it as I am interested to know.

it cant be because my password witch i wished to use had letters & numbers

here I have found the definition for you:

Alphanumeric is a collective term used to identify letters of the Latin alphabet and Arabic digits. There are either 36 (single case) or 62 (case-sensitive) alphanumeric characters. The alphanumeric character set consists of the numbers 0 to 9 and letters A to Z.

In computing terminology, a character stored in alphanumeric form is considerably smaller than storing a 8-bit ASCII character, as each character is only 6 bits in length.

There is no standard for storing 6-bit alphanumeric data. A 6-bit field has 64 possible values, so if only 36 are used in single case, there is room for another 28 characters (usually slashes and other punctuation), making alphanumeric data useful for storing text and website addresses.

Alphanumeric data can be stored even more compactly in computer systems if the storage medium is calculated in base 36, where each numerical position represents a character. Storing characters in base 36 and base 64 (6 bits per character) is more memory efficient for storing text-only data than base 256 (using 8 bits, or a byte, for each character).

(from wikipedia)