High quality models will have firebrick lining in the combustion area. It provides a stove that warms up slowly, retains heat well, does not warp and lasts for many years. Cast iron is relatively brittle and is subject to cracking if handled roughly. For this reason, you should carefully inspect new and used cast-iron units for breaks before using them. Firebrick or ceramic liners may be used in any of the three stove types.
Liners add mass to the stove and tend to spread heat away from hot spots in the fire.
This promotes more uniform heating and prolongs the useful life of the stove. Liners also lengthen the time between the starting of the fire and the heating of the room. Unless you know the conditions the stove was tested under and how these compare to your own situation, most ratings are of little value. At best, ratings should be used only as general guides. Selecting a size based on space available and required clearances around the stove or on capacity to handle the size fuel you have may be more important than rating.