A call this a rock box as it is one way of displaying rockery plants above ground level.
The size of these interesting rock plants allows you to get many species in one small container. This tannalized wooden frame was custom made about 4 feet square and is on display outside the new Alpine house at RHS gardens Harlow Carr. I counted over 30 different species planted in this one container.
Below is a close-up of the Armeria junperfolia from the same display.
See also Rock Gardens in Miniature