What is ericaceous compost used for?

Acidic compost for plants that love acid conditions, like azaleas, rhododendrons, blueberries, camellias, heathers, magnolias--if you've been told to 'acidify the soil' use some ericaceous material, and everything will turn out great! It's apparently primarily a UK term, in the US, just ask for acidic compost/mulching materials.

http://www.uk.gardenweb.com/forums/load/ukgard/msg0312045225513.html

http://www.johninnes.info/ericaceous.htm

http://www.focusdiy.co.uk/invt/182518