A divisional patent application may occur when the patent application according to the examiner or examining office contains more than one invention in the same application. In the USA for instance, one application must be for one invention. As such one inventor can obtain a patent right for one invention only. Sometimes it just so happens that the examiner feels that a specific section of the claims in the application should be for another invention or part of the invention. The examiner will ask which part of the claims he must examine. In essence then one must file a divisional patent application for the other elements or claims which have been noted by the examiner. The filing date can stay the same even when filing the divisional patent application at a later date.
The divisional application may thus be required when the original patent application does not show enough unity in the invention. This means that you have described more than one invention in one application. You will thus have to file a separate application for all the subsequent inventions described in the application. As such the divisional patent is restrictive. One may claim for variations of an invention, but such should all be separate applications. It doesn’t mean one has to resubmit the original patent application; only that one will have to file divisional patent applications for the claims not included in the original patent.
It is not all that complicated at the end of the day. To file a divisional patent application you submit the same drawings and specifications with only the claim group differing. There is the cost for an additional filing that should be considered. It should thus be avoided if you want to save patent application costs. The best is to get the assistance of intellectual property attorneys to ensure that each patent application embodies only one invention.