You can create additional custom facet/filter groups by using what we call "Tag Groups." This feature is similar to those found on several other themes and apps.
Tag Groups are available on search results for Products and Articles (currently the two Shopify entity types that support tags).
If this feature is enabled, any entity with a tag containing the underscore character will auto-generate a custom tag group.
For example, a product that contains the tag Pattern_Waves will generate a facet/filter group titled Pattern with an option of Waves.