The meaning of the Android studio check box in the property window layout editor

Just updated to the latest version (2.2) and saw the check box that can be selected in the properties window. You can choose from three different options, two of which are easy to explain. One is the selected check box and the other is the minus sign. But what does blank represent? I tried all three, but I didn't find any difference between whitespace and minus sign. It may be not only specific to Android studio, but also the first time I used it

resolvent:

The answer may be a little late, but I encountered the same problem when using Android studio. By viewing the text view of the layout XML file, you can find the meaning of the three states of the Boolean attribute check box:

>Select - > set the attribute to "true" > blank - > set the attribute to "false" > minus sign - > the attribute is not used and will not appear in the XML file