1. Selected by the police agency and defense ministry. from people who are eligible for military service.
2. according to military service law, they serve 21 months just like those in active duty in the army. Their ranks are the same.
3. the distinguishing features on their uniform that differentiate them 의무경찰 (right/silver crest on cap) from regular full-time police officers 일반경찰 (left/gold crest on cap)

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Credits:

서울경찰 (Seoul Police): here,
Translated by @snxy,
Shared by TVXQ! Express