Can intersect border
Each structural element that intersects the border of an anchor zone is deleted from the list of available anchors.
This means that solitary vertical or horizontal lines that intersect the anchor zone borders twice are discarded and not listed in the Available structural anchors list. This is true even if the "Can intersect border" option is selected. These kinds of structural elements are unsuitable as anchors because a shift of a document means that the anchor zone causes no change to the anchor but a significant change during zone registration.
The following example shows two structural elements in the zone. If you have disabled the Can intersect border option, the element marked with the red arrow is discarded, and only the element marked with the green arrow is included in the list of Available structural anchors.