Subject classification:

A Compound Subject is a basic subject when it has at least one focus, or has at least an aspect, i.e., it has a basic facet and one or more isolate facets. Agriculture is a basic subject, but agriculture of wheat or diseases of wheat plants are compound subjects. Psychology is a basic subject but child psychology, or personality disorder are compound subjects. The number of compound subjects in this universe is infinite. 

A Complex Subject, on the other hand, is a two phased subject and is formed by the combination of two or more basic or compound. subjects, and made to express the relation between them, but excluding, the case when one of the subjects forms an isolate of the other, formed by subject device. Examples: psychology for nurses; comparative study of Indian and British constitutions; or influence of geography on history, or relation between anatomy and physiology. Such subjects are mostly interdisciplinary. The process of analysing a complex subject into its constituent phases is known a Phase Analysis.