Which is a good use case for a site property?

A site property is particularly useful for storing configuration values that are consistent across different sessions and users, effectively serving as global settings for the application. This enables you to manage settings central to the application environment without hardcoding them, thus improving flexibility and maintainability.

The use case for a REST Web service API key aligns perfectly as it is typically a constant value that should be consistent across all users and sessions of the application. Storing this API key as a site property allows you to easily reference it throughout your application without the risk of exposing it in client-side code or needing to redefine it in multiple locations.

In contrast, values like the current user ID, total quantity of products in stock, or search keywords are dynamic and can change frequently based on user interactions or application state. These values are better suited to be handled as local variables, session variables, or through other mechanisms that ensure they reflect the current context of the user or the application, thus not being ideal candidates for site properties.