Countertop steamers are desgined to either naturally circulate the heat and steam within the cabinet or they are desgiend with a "forced air". Either one is considered a convection steamer. Convection steamers are ideal for smaller to medium duty kitchens. They are small in profile and can be used to steam, poach and stew food. They can also be used to thaw out or reheat food. They can cook food quickly but they are gentle enough to not make them inedible should an operator lose track of time.
Essentially they are the larger version of their countertop cousins. They employ the same technology just on a larger scale. They are ideal for medium to heavy duty kitchen.
Pressure steamers combine the power of steam and pressure to cook larger and denser quantities of food. They cook things quickly and if an operator is not watching they can easily over cook things. They are ideal for larger scale food operations that need to push out large quantities of food quickly.
The perfect combination of a convection steamer and a kettle for cooking sauces, stews and soups. A chef can have vegetables and turkey steaming while cooking gravy in the kettle.