The Web Mask Validator is one of the Validations that you can add to a Column defined in the Database Repository.
The purpose of the Web Mask Validator is to provide masked editing in web pages. Only the Ajax Extension Template (part of the ASP.NET 2.0 Templates) currently has an implementation for this, using the MaskedEditExtender from Microsoft's AjaxControlToolkit. You need to add the Ajax Extension Template to your ASP.NET project before you can see any effect of the Web Mask Validator.
The Web Mask Validator allows you to define a mask for text input.
Using Ajax support, this mask is displayed in the textbox in the web page, ensuring that input is according to the desired pattern.
The mask is added as a child template to the column it applies to. It appears in the Template Tree on the configuration page of the Page Template.
See also
More about the MaskedEdit can be found in the online documentation of the Microsoft ASP.NET Ajax Control Toolkit:
http://ajax.asp.net/ajaxtoolkit/MaskedEdit/MaskedEdit.aspx