From NCSU EDA Wiki
Design Rule Check
An enclosure rule checks the distance between the outside edges of the enclosed shapes to the inside edges of the enclosing shapes. This is commonly used for vias to specify the enclosure of metal around the via cut shape.
Enclosure does not imply that one shape must be completely contained within another. Rather, it merely checks the spacing of the edges.
Enclosure is often confused with Overlap, which refers to the distance between inside edges of overlaping shapes.