四分律行事鈔資持記 1.1126

1.1126

明會夏中。恐謂安居必身在界，不開明會，故須問決。答引會衣，例同會夏。彼律：衣界不立勢分，故須身分入內方成。

In [the section on] explaining the dawn-meeting during the rains retreat: Fearing it might be thought that for the rains retreat one's physical body must remain within the boundary and no dawn-meeting is permitted, it was necessary to raise a question and resolve it. The answer cites the dawn-meeting with robes as an analogy for the dawn-meeting during the retreat. In that Vinaya, no buffer zone (upacāra) is established for the robe boundary; therefore, a part of the physical body must enter within [the boundary] for it to be accomplished.