Hi,
That field is not currently used. It was originally intended for indicating if a user is approved to post in the forums but it has never been implemented so that field is not surfaced to the UI anywhere.
If you want to block sign in until the email is confirmed we use the RegisterConfirmGuid field. If it has a guid in it other than null or empty guid then the email has not been confirmed. We set the guid and set IsLockedOut to true, send an email that includes the guid and they can click a link to verify and the guid gets set back to null or empty guid and the account is unlocked.
The IsLockedOut field can be set to false to prevent a user from signing in.
SiteLogin inherits from an ASP.NET Login control so some of the magic is hidden in that base class, I'm sure it is calling the membershipprovider.ValidateUser method under the hood somewhere.
Hope it helps,
Joe