You can use driven ("ad" type) and reference dimensions on the right side of relations only (to drive the value of other dimensions or parameters).

If you have used a driven dimension in a relation to drive a model dimension, the model dimension also changes until you regenerate the model in Drawing mode. You can use reference dimensions to drive model dimensions because they regenerate in Part or Assembly mode; however, you should not use driven dimensions to drive model dimensions because incorrect model dimensions might occur if you change the model in Part or Assembly mode. To add relations, use the Tools > Relations command.