A claim's subject-matter is normally defined in terms of positive features indicating that certain technical elements are present. Exceptionally, however, the subject-matter may be restricted using a negative limitation expressly stating that particular features are absent. This may be done e.g. if the absence of a feature can be deduced from the application as filed (see T 278/88).
Negative limitations such as disclaimers may be used only if adding positive features to the claim either would not define more clearly and concisely the subject-matter still protectable (see G 1/03 and T 4/80) or would unduly limit the scope of the claim (see T 1050/93). It has to be clear what is excluded by means of the disclaimer (see T 286/06). A claim containing one or more disclaimers must also fully comply with the clarity and conciseness requirements of Art. 84 (see G 1/03, Reasons 3). Moreover, in the interests of the patent's transparency, the excluded prior art needs to be indicated in the description in accordance with Rule 42(1)(b), and the relation between the prior art and the disclaimer needs to be shown.
For the allowability of disclaimers excluding embodiments that were disclosed in the original application as being part of the invention, see H‑V, 4.2. With respect to the allowability of disclaimers not disclosed in the application as originally filed (so-called undisclosed disclaimers), see H‑V, 4.1.