What does 3 check box appearances in Site Manager site options signify ??

[Partha]

Hi bigstar, when I go to Sites --> Site Manager and then pick a FTP site of my choice from the left column site list , and then go to the "Options" tab in the right, if I click once, the check box becomes blank, if I click again, a tick appears in the box, and I click thrice, it returns to it's normal default full green colored box . What does these three check box appearances signify ?? TIA .

Partha