Different designers of a structure exchange information on their plans (models) with each other. This information is exchanged by reference data which may be in another format than what the designer's application is using. To facilitate a designer to maintain correspondence between his/her model and models handled by different designers, when the reference data is read and an entity is recognized in the data, geometric characteristics of the entity are calculated, and an identifier is formed for the entity by means of the geometric characteristics. The identifier is stored in the model, and it can be used to identify the entity as long as the entity remains unchanged although reference data is updated.