When buying a cast iron stove, multifuel / multi-fuel should mean that is comes with a coal grate and is initially set up for the burning of coal. You can also burn wood, but it is best to make a few changes.
Coal and smokeless fuel should be burned on a coal grate. This is because coal needs air to be able to get underneath for it to burn from the bottom up. However, wood should be burned on the base of a stove on a bed of ash or sand as you want wood to burn from the top down. The ash or sand is to insulate the base of the stove.
For burning wood only in a multifuel stove, remove the coal grate, then put in the bed of ash or sand. The removing of the grate means that you are left with a much bigger firebox which will hold a greater capacity of wood. Some multifuel stoves also come with a log retainer bar. This is a very useful thing to have, as it prevents logs rolling forwards when the coal grate has been taken out.
When burning coal or smokeless fuel, use your bottom air vent as a booster and and when required. The bottom air vent (apart from during initial lighting stages) should not be used when burning wood. This is because you don't want to be letting air underneath wood when burning as it should burn from the top only.