Sir Thomas Innes of Learney, in his great work Scots Heraldry, wrote the following:

The Compartment
In Scotland, supporters are always depicted on a suitable mount, rocks or maybe seashore, but occasionally the grant lays down some special feature; thus the Earl of Perth's supporters are set on "a compartment strewn with caltraps", and those of Dundas of that Ilk set on "a salamander in flames of fire". Nisbet restricts compartments to the Baronage, but they are occasionally granted as a distinction or for special services. They represent the bearer's territories, and are thus feudal honours, now only assigned to historic territorial houses, and in the case of clan-chiefs, now usually incorporate the plant-badge.