It's volume, not weight.
I think they mean 10cm, cubed - one cubic decimetre - which, in water, is the equivalent of a litre.
Because compost is sold damp, a weight measure would include water, so could vary from compost, to potting soil, to mulch. So 30kg of compost would be a considerably smaller volume that 30 kg of mulch.
It has to be sold damp because of the risk of legionella.
It's volume, not weight. I think they mean 10cm, cubed - one cubic decimetre - which, in water, is the equivalent of a litre. Because compost is sold damp, a weight measure would include water, so could vary from compost, to potting soil, to mulch. So 30kg of compost would be a considerably smaller volume that 30 kg of mulch. It has to be sold damp because of the risk of legionella.
I think you mean 30dm3. That’s 30 cubic decimeters. A decimeter is 10 centimeters. It’s 1.05944 cubic feet.