The juàn reading is reserved for animal enclosures: 猪圈 (pigsty), 羊圈 (sheepfold), 圈养 (rear in captivity). If the 'enclosure' is metaphorical (social, geometric) use quān; if it's a literal pen for livestock use juàn.
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Dictionaries also list a verb reading juān ('to confine, to lock up') tied to the same pen-imagery, but in everyday usage the verb sense has merged into juàn (pen up livestock) or quān (circle off, cordon). Modern speakers rarely produce the juān reading.
Outer enclosure radical (wéi) — wraps the phonetic to depict the fenced boundary of a livestock pen. The frame is the meaning here: a closed perimeter holding animals inside.
Inner 卷 supplies the sound. The juàn reading preserves the phonetic more faithfully than the quān reading does — the tone shifted but the rime stayed close to juǎn.
