One of the most advanced and practically significant approaches in constraint programming is the method of subdefinite models.

This method is the basis for a multilevel constraint programming tech- nology that has been developed to solve various problems in economics, engineering, calendar planning, etc.

A subdefinite model is a pair (*X*, *C*), where *X* is a set of variables and
*C* is a set of constraints on them. In the subdefinite model, the values of
variables can be both precise and subdefinite. For variables of numerical
data types subdefinite values can be defined as intervals or sets of possible
values of variables.

We call a subdefinite model, where subdefinite values of variables may be only numeric intervals, an interval model.

This paper deals with some problems related to the representation of interval models and interval data in the UniCalc solver intended to solve the systems of algebraic equations and ineualities. A number of methods developed by the authors for the solution of these problems in the UniCalc environment are considered.