Yes, I stumbled against it on the MS knowledge base.
MS states that is for internal use only.
You will see them appearing if you migrated from 2.6 to 5.0.
MS says it is used to speed up the creation of forms.

You can find it at http://support.microsoft.com/support/kb/articles/q136/9/24.asp
Visual FoxPro 3.0 Undocumented Properties