Domain is the representative of the

Domain is the representative of the entire teaching domain and is composed of a number of different concepts (represented by circles in Fig. 1). An important feature about concepts in a domain is that they are not isolated but are related to one another in various ways. The concepts within the domain may be interrelated in two ways
 If it is necessary to learn a particular concept before learning the other concept, then it is said that former concept is prerequisite for later concept and this relation is shown with full arrow.
 Sometimes the essential prerequisite relation exists between two concepts only to some extent and this type of relationship between concepts is shown as dashed arrow.