וְעָשִׂ֧יתָ עַל־הַחֹ֛שֶׁן שַֽׁרְשֹׁ֥ת גַּבְלֻ֖ת מַעֲשֵׂ֣ה עֲבֹ֑ת זָהָ֖ב טָהֽוֹר׃
Rashi on Exodus
על החשן means [AND THOU SHALT MAKE] FOR THE BREAST-PLATE (not upon it), — in order to fasten them (the plaited chains) on its rings, as is explained further on in the section.
Sforno on Exodus
גבלות, not in order to lengthen or shorten but precisely for a short distance between the top of the breastplate and the forward part of the shoulder pad as described in verse 14 merely to link these two sections of the High Priest’s garments.
Rashbam on Exodus
ועשית על החשן שרשות גבלות, these are the rope-like chains described earlier inverse 14.