Hi ,
First regarding a Sort.
A Sort using the menu options in the Ribbon is like any other Excel user operation using the User Interface ; it can be undone using Undo or pressing the CTRL Z keys combination.
Clicking on the Clear button in the Ribbon removes all filters which had been applied.
The ReApply command comes into the picture only if you have already applied a Filter , and thereafter added data ; this new data might or might fall in the criteria using which the filter has been applied.
In order to have the already applied filter now apply to the freshly added data , you click on the ReApply button , which now applies the filter to the extended range , inclusive of the added data.
If no new data has been added , the ReApply command does nothing extra.
Narayan