Based on the native “Recent Posts” widget, this one displays the recent posts accord the root category of the current post (or category).
You will have to define a main and secondary category.
By default, the main one is used to display the latest posts. If visitors are not currently in the secondary (or sub-categories), then it is this one which is used.
This widget is interresting when you have 2 different categories (with no linked subject) and you want to display the latest posts of the other category, rather than display all (or from the current one).
The parameters are the same as the native widget, with the addition of :
You will have to enter categories ID (only one per field).