瑜伽師地論略纂 8.34

8.34

論「僧伽胝」者，此云上衣，即上着之衣。如外垣墻，此衣亦爾，最在外着，最向上着，名為上衣。

Regarding the treatise's "saṃghāṭī": here it is translated as "upper robe" (shangyi), that is, a robe worn on top. Just like an outer wall, this robe is likewise: it is worn outermost and worn uppermost, hence it is called the "upper robe."