Check box for Do not apply visual spell-checking

[elmpine]

This has been bugging me for some time:
The Filemaker documentation describes on and off - however the checkbox has 3 options: blank, checked, and line.
What is the line option for? (I can only think of Schrodinger's dictionary)

[bdbd]

Line means you have multiple layout objects selected and the objects are set to a mix of on and off.

[elmpine]

Just checked - definitely only one item selected.

[Malcolm]

That single object may be a group of objects.

[elmpine]

Multiple selection is possible, and the line would be appropriate.
But it appears even on only one field (see image):

It's something that I have seen for sometime (years) but never understood.