circle, world
畍 centers on a bounded area or realm: a circle as a defined space, and by extension the world as the sphere of human activity.
畍 is an older variant of 界. Both combine 田 (field) with a phonetic component (介 in 畍, 介 in 界), expressing the idea of boundaries between fields and, by extension, the world as a bounded realm.
A field 田 divided by a person 介 standing between plots: the boundary line creates a circle or world of its own.
For カイ, imagine a kite flying over the field, its string marking the boundary of the world below: kite -> カイ.