Reactor Convex and Concave objects

Reactor classifies objects into two types: convex and concave. This affects the way bodies collide with other bodies, and the accuracy of the simulation. In every Reactor help reference that shipped with Max have a good exploration what often skiped from users. Also in most web tutorials covering Reactor not sensibly illustration where and why used conver or concave type. But this is important base part to get started that not recommendable to skiped. So here I'll just quote from Reactor reference one comparatively short and good exploration of the essence of this 2 base types:

An object is defined as convex if, given any two points inside the object, you can always go in a straight line from one to the other without going out of the object. Convex objects include spheres, cylinders, and boxes. If an object is not convex, then it must be concave. Concave objects include teapots and (most) landscapes/terrains and rooms.Generally, convex objects are faster to simulate than concave objects. Because of this, you should aim to use convex objects as often as possible for simulation. reactor enables you to treat concave objects as convex so that you can take advantage of their faster processing time. However, there are also cases where it is appropriate to treat a convex object as concave. In reactor convex objects have an "inside" which determines when objects are contained within one another, but concave objects do not, and so an object entirely within a concave object will not be considered as interpenetrating with that object. You cannot place objects inside a convex object. Instead, you must define an object as concave in order to place objects within it. For example, if you were animating a room, you could design it as a box and treat it as concave, so that you could place objects within it.