When is a divisional application typically filed?
A divisional application is often filed in response to a restriction requirement made by the patent examiner. As stated in the MPEP:
A divisional application is often filed as a result of a restriction requirement made by the examiner.
When an examiner determines that an application contains multiple independent and distinct inventions, they may issue a restriction requirement. The applicant can then elect one invention to pursue in the original application and file a divisional application to pursue the non-elected invention(s).
To learn more:
Topics:
MPEP 200 - Types and Status of Application; Benefit and Priority,
Patent Law,
Patent Procedure