Possible self-reference to a hierarchical parent item.

public virtual sealed Guid ParentItemId

Remarks

Items with a self reference to a parent item are special cases.
For instance, an item may be a modifier for another item.
Perhaps someone purchased a pair of pants (which would be one line item)
and had some custom work done on them (which would be another line item
linked to the first one).

See also:

Class LineItemEntity