VS 2010 SQL Entity Framework - Nullable None

I am currently setting up a database table using the entity framework model.

When I add a binary option, to my surprise the nullable property field has three possible values true, false & (none).

Whilst incan understand what true and false mean, i.e cna the field contain null values, there is the other option of none.

I am unsure what none means as the field can eithe contain null values or not ?

Any help would be appreciated.