Advanced tab on Model Options dialog.
Component blocks that are considered to be flat (i.e. consist of 2 large surfaces and several smaller 'edge' surfaces) are normally represented in the simulations by a single shading surface. The Maximum flat surface thickness entered here defines the thickness of component blocks below which they are considered to be "flat" and therefore to be modelled using a single shading surface. This same threshold value is used for all component blocks in the model. The default threshold value is 0.1m.
The method used to define which component block surface(s) are used to represent flat component blocks can be entered on the Construction tab under the Component Block header using the Flat surface position option.