Firstly, you need to understand the difference between burning wood and burning multi fuels (mineral or solid fuels) such as anthracite or smokeless coal nuggets. To burn efficiently multi fuels require combustion air from underneath the fuel load (known as primary air, with this air control generally being at the bottom of the stove) – hence the open grate feature to let the air through to the fuel. Wood takes its combustion air from the top (secondary air, generally with the air control at the top of the stove) with the wood load burning from the top downwards. Wood fuel can therefore sit and burn effectively on a flat base so that stoves which are designated as woodburning stoves will either have a small grate or simply no grate at all, which easily allows the build-up of ash to create a heat-reflecting bed to help the wood burn better and protect the stove's base.
Multi fuel stoves are designed to work well burning either wood or multi fuel. Their CE Tests (usually for wood and Ancit) show that there isn't any real trade-off in efficiency between the two fuel types for this compromise. Some of the grates on multi fuel stoves, like the Dunsley Highlander range and the Olymberyl® Baby Gabriel® can be opened and closed (also doubling up as external riddlers), to allow you to create the flat base and ash bed for wood or alternatively the open grate required by multi fuels.