The rule is that a patent can claim only one invention.
If, during the search procedure, the EPO's patent examiner identifies more than one invention in an application, he will inform the applicant that there is non-unity of invention and that he can file (within a time limit) a further application that defines and delimits the other invention (or several further applications, as the case may be).
The first filed application is known as the "parent" application and all other applications derived from the parent are called "divisionals". There can be several generations of divisional applications (see OJ EPO 2014 , A22). A divisional does not introduce new technical content. And it will feature in the same simple patent family as the parent application.
A divisional can be filed as long as an earlier (parent) European or Euro-PCT application is pending. The priority is based on the filing date of the parent application.
Note: Divisionals are published as a rule four months after their filing date.
Comprehensive details of the different application types appear on the EPO website under DOCDB simple patent family and INPADOC extended patent family .