You get redirected (302) to the application status page when you request a page/URL of your Sitefinity application during initialization/upgrade stage of the application. The reason for redirecting to the application status page, instead of just rendering it on the same URL you requested, is to tell the outside systems which interact with your application (ex: search engine crawlers) that the requested URL is temporarily unavailable, but it will be available shortly. By saying this, the outside systems should not do anything with the response received from this request. For example, the search engine crawler will know that the requested URL is temporarily redirected to a different one and it should not index this response, but instead, try again later.
However, the application status page response code is 200. While this is not a problem for the search engine crawlers, if you are calling a public REST service, which is served by your Sitefinity application, from a third party application, you will not see the redirect to the Application status page (302) and you will get the final response status code which is 200, which means the service returned the expected content. But this is not the case, since the 200 status code is the response status code of the application status page and the response contains the application status code HTML. This behavior may cause unexpected results in your third party application. To get around this problem you have the option to change the response code of the Application status page to be different than 200. You can do this by adding a new parameter in the <appSettings> of the web.config with key sf:AppStatusPageResponseCode and value of the response status code you want.
Example: <add key="sf:AppStatusPageResponseCode" value="503" />
<add key="sf:AppStatusPageResponseCode" value="503" />
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