The Wood Ash is an item from the Witchery mod. This item is used to create many items from Witchery, such as Quicklime, Ritual Chalk, and Mutandis. It is also used to create Bone Meal more efficiently than with Vanilla's recipe.
If the smelt all saplings to Wood Ash option is enabled, all treeSapling items will be able to be smelted into Wood Ash. Otherwise, only Vanilla and Witchery saplings can be used.
There is only a chance that fumes will be created with a Witches Oven.
The second output is dependent on the sapling, and will always be a fume. Since any sapling can be used in this recipe, this output is not listed.