A European application must "relate to one invention only or to a group of inventions so linked as to form a single general inventive concept". The second of these alternatives, i.e. the single-concept linked group, may give rise to a plurality of independent claims in the same category provided these claims comply with Rule 43(2) (see F-IV, 3.2 and F-IV, 3.3), but the more usual case is a plurality of independent claims in different categories.
The requirement of unity of invention serves a regulatory function in the interests of an efficient procedure up to grant (see T 110/82). It would be inappropriate to accept those applications which, because of their heterogeneous content, entail a far greater than average expense to process, especially in respect of the search, since this expense must partly be borne by the fees levied for other applications. A further aspect is the requirement as to ready comprehensibility of the subject-matter of the application, which may be impaired by heterogeneous subject-matter.
When determining unity of invention, a finding of lack of clarity of the claims is on its own not sufficient grounds for a finding of lack of unity.
Normally, too, the sequence of the claims should not have an impact on the determination of unity of invention. However, it will have an impact on which invention is to be considered the first invention mentioned in the claims (see F-V, 8.2).
Moreover, the fact that the claimed separate inventions belong to different groups of the classification is not in itself a reason for a finding of lack of unity.
An application may contain claims of different categories, or several independent claims of the same category. This is not in itself a reason for an objection of lack of unity of invention if the requirements of Art. 82 and Rule 44 are otherwise met (the relationship between Rule 43(2) and Art. 82 is explained in more detail in F-V, 14).
With regard to substantive criteria, unity of invention is examined in search and substantive examination in both European and PCT procedures according to the same principles. This does not apply to the respective procedures themselves, where significant differences exist.