A BinaryMilestone is a single Yes/No question or True/False statement. The MileStone? has been reached when the answer is "Yes" or the statement is true.
In one project where forced to use MicrosoftProject for planning, I maintained the project plan with all tasks marked as either 0% complete or 100% complete at all times. I'd expected to get some serious stick from senior management over this, but they either didn't notice or thought it was just fine. -- JamesCrook
With regard to people who say that they are "almost done" - ask them "Would you rather be almost alive, or almost dead?" In project management "almost" should be a synonym for not. -- ScottJohnson
See also TaskCompleteDefinition.