It is the authorization key that the Connect to Root Deliveries flow stores for you; Sprwt sends it with every request to Root Deliveries. It is a private integration field, so it is skipped entirely for regular admins: the Manually Configure button on the Root Deliveries integration tile (where the field would appear) is itself only shown to super admins, and the field is also omitted from the normal Business Settings configuration page. Super admins can see it there, and also in the Configs group of the system Config page (/admin/config, which is itself super-admin only). Normal admins never need it — use Automatically Configure instead. The related Client Id and Client Secret are hidden system values managed by Sprwt (not editable through the UI).