"No patent practitioner can ever know"?
The moral: no patent practitioner can ever know what terms must be expressly defined, and the only alternative is to accept a "zone of uncertainty" regarding the scope or validity of patent claims or to burden every specification withevery definition of every term that could possibly be relevant in future. In short, both an impractical and impossible standard to hold either patents or patent practitioners to satisfy.
The facts of Teva v. Sandoz do not illustrate this point. The critical parameter in the Teva invention WAS the molecular weight range. It was foreseeable that claims to a particular molecular weight range would be of commercial significance. It was equally foreseeable that establishing related claim elements to define the range would be important.
Yet, Teva had claims speaking of average molecular weight without defining how average molecular weight is measured. The deficiency became apparent in prosecution. Teva reached for a straw by invoking peak average molecular weight (Mp), which is a mode, not an average. Mp is not a species of the genus "average molecular weight."