This paper proposes an expert system description based on a feature diagram tree, annotated with weighted variant features in the software family context. The principle of how some variant features may be described on the basis of fuzzy logic is presented and fuzzy probability of market conditions is introduced. The proposed description integrates the external cross-tree constraints, and the resulting homogeneity of the logic then allows its usage to further applications, e.g., generators, or to the purposes of constraint based reasoning itself. The appliance of the proposed approach is demonstrated by the example of a mobile phone tariff description. In the example descriptions of the decision tree for customers have been prepared.