Black sleeves are made of stretchy mesh fabric and faux leather.  The main measurements for the shrug are shoulder width and upper arm. You can wear those as an accessory or if it's a bit chill and take off later if it's getting warm. The sleeves are quite small when folded so there's no problem putting it in a backpack, purse, or even a pocket.

FABRIC Faux leather - PES 97%, EL 3% Mesh: PES 100%