Where the invention relates to a product, it may be defined in a claim in various ways, viz. as a chemical product by its chemical formula, as a product of a process (if no clearer definition is possible; see also F‑IV, 4.12) or, exceptionally, by its parameters.
Parameters are characteristic values, which may be values of directly measurable properties (e.g. the melting point of a substance, the flexural strength of a steel, the resistance of an electrical conductor) or may be defined as more or less complicated mathematical combinations of several variables in the form of formulae.
Characterisation of a product mainly by its parameters should only be allowed in those cases where the invention cannot be adequately defined in any other way, provided that those parameters can be clearly and reliably determined either by indications in the description or by objective procedures which are usual in the art (see T 94/82). The same applies to a process-related feature which is defined by parameters. Cases in which unusual parameters are employed or a non-accessible apparatus for measuring the parameter(s) is used are prima facie objectionable on grounds of lack of clarity, as no meaningful comparison with the prior art can be made. Such cases might also disguise lack of novelty (see G‑VI, 6).
Use of unusual parameters may however be allowable if it is evident from the application that the skilled person would face no difficulty in carrying out the presented tests and would thereby be able to establish the exact meaning of the parameter and to make a meaningful comparison with the prior art (T 231/01). In addition, the onus of proof that an unusual parameter is a genuine distinctive feature vis-à-vis the prior art lies with the applicant. No benefit of doubt can be accorded in this respect (see G‑VI, 6).
Whether the method of and the means for measurement of the parameters need also be in the claim is dealt with in F‑IV, 4.18. For further issues relating to clarity, lack of support and sufficiency of disclosure regarding parameters, see F‑III, 11, and F‑IV, 6.4.