Non-standard reference masses, specified as additional or alternative mass definitions or payload states, if technically necessary, shall be defined and justified in the technical specification or in the application standard.In such cases the mass and payload definitions set out in 4.4 and 4.5 should be used as a basis wherever possible. This can be achieved for example by applying factors or by including a specific mass or payload increment. In such cases, a suitable notation should be defined following the principles of the system set out in 3.2. For this notation, the unused letters of the alphabet are available.