See ISPE Guidelines 5.11. If there is a large number of different embodiments, only the reference signs of the most important embodiments need be incorporated in the independent claim(s).
If text is added to reference signs in parentheses in the claims, lack of clarity can arise (Art. 6). Expressions such as "securing means (screw 13, nail 14)" or "valve assembly (valve seat 23, valve element 27, valve seat 28)" are not reference signs in the sense of Rule 6.2(b) but are special features. It is unclear whether the features added to the reference signs are limiting or not. Accordingly, such bracketed features are generally not permissible. However, additional references to those figures, where particular reference signs are to be found, such as "(13 - Figure 3; 14 - Figure 4)", are unobjectionable.
A lack of clarity can also arise with bracketed expressions that do not include reference signs, e.g. "(concrete) moulded brick". In contrast, bracketed expressions with a generally accepted meaning are allowable, e.g. "(meth)acrylate" which is known as an abbreviation for "acrylate and methacrylate". The use of brackets in chemical or mathematical formulae is also unobjectionable. The use of brackets for providing physical values complying with the requirements of Rule 10.1 is unobjectionable as well.