四分律行事鈔資持記 3.87

3.87

七中。引《經》示褊袒通肩之意。律中，佛上座前方袒，可驗餘時通披。經中未詳何經，謂對佛僧不恭敬故。

In the seventh [part], it cites a sūtra to show the intent behind baring one shoulder versus covering both shoulders. In the Vinaya, only when the Buddha or senior monks were in front did one bare the shoulder; from this it can be verified that at other times it was draped over both shoulders. In the sūtra text, it is not clear which sūtra it is; it means that doing otherwise shows a lack of respect before the Buddha and Saṅgha.