"Is Available" will display anytime you have an open slot for the selected date regardless of current prospects in New, Lead, & Pursued statuses. "May Be Available" will display anytime you have an open slot for the selected date but have a current prospect in the Pending status.
Confirmed will display "not available".
Use the urgent message feature if you would like to convey to a
prospect that a date has limited availability. Click Web Tools >
Configuration > Date Checker. Locate "Date Available Options" section.
Enter text in the "Date Available 'Congratulations Page' Urgent Test" field. You
can type something like, "Do not delay in requesting information, we have only
one system left for your date."