A divisional application is one that is a spin-off if a patent application that claims subject matter that is fully supported in a previous patent application’s specification and drawings if required but is not claimed in the parent application. The divisional gets the benefit of the filing date of the earliest relevant parent patent application.
An example would be an application that discussed two inventive ways to make a toaster that toasted bread to desired degree automatically. One uses a camera to look at the bread and pops the toaster when a set degree of brown is detected and the other uses an electronic “nose” to sense the vapors from burning the bread. Both are well supported with details in the specification and drawings.
If there is one claim to one invention and one claim to the other invention the examiner will declare that you do not have “unity of invention” and require you to pick between them and cancel any claims to the other. In the U.S. this has different name with slightly different criteria.
If you want to pursue the unselected invention it will be via a divisional application. It is a new application that states that it is a divisional of patent application 1235667 and claims priority from it.
It could also be a case where two inventions are described but only one is claimed. You not get any notice from the examiner but can later chose to file a divisional.
There are rules as to when a divisional can be filed and these have been recently changed in Mexico. A rule that many jurisdictions have is that it can only be filed while the parent application is still pending, Mexico tightened up those rules.