As per Article 82 of EPC, the “European patent application shall relate to one invention only or to a group of inventions so linked as to form a single general inventive concept”.
The usual reason for filing a European divisional application is that the parent application does not satisfy the requirements as to unity of invention (Article 82) and the applicant is not content with limiting it.
All the states designated in the parent application at the time of filing of the divisional application are deemed to be designated in the latter. However, contracting states the designations of which have been withdrawn or are deemed to be withdrawn in respect of the parent application at the time of filing the divisional application cannot be designated in respect of the divisional application. The same applies to extension and validation states.