Compartment
A compartment is the ground drawn under the shield for the supporters to stand on. It is usually a grassy mound, rocks or some other piece of landscape. It is sometimes said to stand for the land held by the bearer of the arms.
The UK royal arms stand on a grassy mound with thistles, Tudor roses and shamrocks for Scotland, England and Ireland. If a compartment is written into the blazon it is part of the grant. If not, the artist may draw what they like: the wattle under the arms of Australia is customary but not required. Compartments usually arrive with supporters.
The coat of arms maker has no ready-made compartment, but you can build one. Upload your own image of a mound or rocks on the Uploads tab, which stays in your browser, or layer plants like thistles and roses under the shield. The parts of a coat of arms guide shows where it sits.