What is this directory?

In this directory, Akeeba Backup stores the stacked optional filters (Optional Filters in the Configuration page). Unlike regular filters which are always loaded, optional filters are only loaded when the user chooses to enable them. Each filter consists of two files: filtername.ini and filtername.php. The former contains the filter-specific configuration options and the later contains the actual filter code.

If you want to create new optional filters, put them in here. Do note that the INI file must always contain a boolean key named core.filters.filtername.enabled which controls the loading of this particular filter.

Optional filters are always named Akeeba\Engine\Filter\StackFiltername so that the autoloader can find them.