That’s chlorinated isocyanurate tablets
what type it is:
product: chlorinated isocyanurate tablets
common name: tcca tablets or chlorine tablets
type: stabilized chlorine disinfectant / pool sanitizer
form: solid tablets - usually 200g or 20g tablets
pack: 10kg white bucket with blue lid
above it i can also see aquatabs = troclosene sodium - another stabilized chlorine.
what it's used for:
main use: swimming pools
1. daily chlorination - slow dissolving tablets for skimmers, floaters, or feeders
2. stabilized chlorine - contains cyanuric acid "stabilizer" so sun/uv doesn’t burn off chlorine fast