Client apps key
This is key is required for working with the Yandex.Checkout libraries (right now, there's only one of them: mobile SDK).
You can get your key in the Store settings section of your Merchant Profile. If you run several stores, each one will require a separate key.
The button for issuing the key is disabled by default. You can enable it by making a request to your Yandex.Checkout manager.
The key works just like the secret key: copy it, save it in a safe place, and activate it with a text message password.