四分律刪繁補闕行事鈔 1.117

1.117

若論自然，隨處遠近，則有四別：謂聚落、蘭若、道行、水界。皆不為物造，隨人所居，即有分局，故曰自然。當律無文，諸部詳用。

If discussing natural boundaries (akṛtasīmā), according to the distance of places, there are four distinctions: namely, village, araṇya, traveling on the road, and water boundary. None are constructed for an object; according to where people dwell, delimitations naturally arise; hence they are called 'natural'. Our vinaya contains no explicit text on this, but the various schools use them in detail.