We have added a sub-collection feature allowing you to create a collection tree.
Drag and drop to create collection tree
Go to Apps → Smart Product Filter → Click Edit Filter button
Click Config icon.
Note: This feature only works with Filter Options whose Type is Collection
Drag and drop collection list to build a collection tree
You can also remove collections by clicking X icon. The collections will be moved to “Unselected collections” and they will not show in the front end.
You can also add the unselected collection back by clicking on + icon. The collections will be added to the end of the collection tree. You can move it to the position you would like. 
Save and check the result