A patent can only claim one invention. Sometimes during the course of examination, an Examiner identifies more than one invention in the claims. In that instance, a divisional application can be filed for the second invention. The divisional receives the same filing date as the parent.
You can add new material in a divisional, although any claims based on the new material will not be given the priority date of the parent.
At any time during the lifetime of the application, you can convert a normal application into a divisional application by amending the patent request. However, you cannot convert a patent after it has been granted. A divisional must be filed no later than three months after publication of acceptance of the parent application or, if the parent is never accepted, before the parent application lapses. The divisional must include at least one claim that is supported by the material in the parent.
Even if a divisional is filed while the parent is still pending, it cannot be converted if the proposed parent lapses or is accepted in the meantime.
In the case of Magnum Magnetics Corporation  APO 3, Magnum missed the deadline to a file a divisional. Magnum then required a 22 month extension to the conversion deadline.
In order to obtain this extension, Magnum needed to establish that it had failed to claim divisional status (by conversion or at filing) as a result of an error or omission that occurred before the deadline.
Magnum had used more than one US attorney to file its patent applications. It was not able to obtain any evidence from its original US attorney about the error or omission. Without this evidence, the Hearing Officer could not allow the extension of time (under section 223 of the Australian Patents Act 1990).
The extension request was denied and Magnum was left with claims that were invalid over its own earlier application.
This problem seems to most often arise in situations where a US applicant files two PCT applications for similar inventions. The PCT system cannot be used directly to achieve the effect of a divisional application, or of a US continuation or continuation-in-part application. Article 4 of the Paris Convention permits an international priority claim only from the very first application filed in respect of an invention. So divisionals must be filed at the national level.
In some cases, it is better to file a patent of addition, rather than a divisional. A patent of addition can be used to protect an invention that is novel, but does not involve an inventive step, over an invention disclosed in an earlier application by the same applicant, which would otherwise make it unpatentable if it were the subject of a normal application.