Peter Siedlaczek's String Essentials by Best Service is a Virtual Instrument Audio Plugin and a Standalone Application and Soundware (e.g. samples or presets that load into other products) for macOS and Windows. It includes, and is therefore "powered by", Kontakt Player, which functions as a VST Plugin, an Audio Units Plugin, an AAX Plugin and a Standalone Application.
The OS and Format icons below are for the latest version of Kontakt Player. The version numbers are for Peter Siedlaczek's String Essentials.