Во-первых, CCCCCCCCCC.

Во-вторых,

Help0 Digit (0 to 9, entry required, plus [+] and minus [–] signs not allowed).
9 Digit or space (entry not required, plus and minus signs not allowed).
# Digit or space (entry not required; spaces are displayed as blanks while in Edit mode, but blanks are removed when data is saved; plus and minus signs allowed).
L Letter (A to Z, entry required).
? Letter (A to Z, entry optional).
A Letter or digit (entry required).
a Letter or digit (entry optional).
& Any character or a space (entry required).
C Any character or a space (entry optional).
. , : ; - / Decimal placeholder and thousand, date, and time separators. (The actual character used depends on the settings in the Regional Settings Properties dialog box in Windows Control Panel).
< Causes all characters to be converted to lowercase.
> Causes all characters to be converted to uppercase.
! Causes the input mask to display from right to left, rather than from left to right. Characters typed into the mask always fill it from left to right. You can include the exclamation point anywhere in the input mask.
\ Causes the character that follows to be displayed as the literal character (for example, \A is displayed as just A).