А subject domain consists of objects, connections, and a set of mass problems of a subject domain. Instances of objects are connected with instances of connections between corresponding objects. A notion of a state of connections of a subject domain is introduced as a set of instances of connections between instances of objects of that subject domain. Two classes of connections were introduced – those, instances of which do not change with time – fraternal connections, which define structure of a subject domain and its properties; and those, instances of which change with time, but which server to solve specific mass problems in the subject domain – logical connections. A stability function of a connection was defined – a probability that an instance of a connection will not be destroyed during next change of a state of a subject domain. An inevitability of a connection was defined – a probability that an instance of a given connection will be created during next change of a state of a subject domain. Each of those functions allow classifying connections between objects into three classes: connections with low values of the function, connections with normal values of the function, connections with high values of the function. Numeric boundaries of those classes can be determined based on the distribution of those values. The given classification allow dividing connections into two groups: those that appear more often and those that appear less often; those, that exist during long time and those that exist during shorter time.Introduced notions can be used for a stochastic analysis of subject domains and for optimization of their work.