When a mask is defined, placeholders are defined by the PromptChar property. When inputting data, the user can only replace a placeholder with a character that is of the same type as the one specified in the mask. If the user enters an invalid character, the control rejects the character and generates the XamMaskedInput.InvalidChar event. The control can distinguish between numeric and alphabetic characters for validation, as well as validate for valid content for certain types like, date or time or numeric types such as the correct month or time of day etc...

For a complete list of the various mask tokens as well as examples please refer to Masks topic.

Note: When specifying the mask from within XAML and using one of the special tokens that are enclosed within {}, you must preceed the mask with {} - e.g. {}{date}.

Reference