袋 doubles as a measure word: 一袋米 (a bag of rice), 两袋面粉 (two bags of flour). Common compounds: 口袋 (kǒudài, pocket), 塑料袋 (sùliào dài, plastic bag), 袋子 (dàizi, bag).
grammar
As a measure word, 袋 directly follows the number: 一袋盐 (a bag of salt). As a noun, it needs its own measure word: 一个袋子 (a bag).
Bottom indexing clothing radical 衣 (Kangxi #145), pictograph of a robe with collar and sleeves. Anchors 袋 in the cloth-and-fabric family alongside 装, 裂, 裤, 袜. A bag was originally just a piece of cloth folded and tied — hence indexed under clothing.
Top component supplies the sound: 代 dài → dài (exact match, no shift). The 'substitute / replace' sense of 代 also adds a faint flavour: a sack stands in for whatever you'd normally carry by hand — a substitute container. Same phonetic 代 in 贷 dài (loan), 玳 dài (tortoiseshell).
