Custom pages for 5XX errors


I would like to upgrade to “pro” plan to exploit the custom pages functionality.
My application is managed by a company which also deals with the hosting of the app. This company is already displaying a custom page for 5XX errors which are not handled by the application. But their SEP (single error page) is not customizable.
My question is the following: Are the custom pages set in Cloudflare will be displayed in priority to the customer ?

Thank you in advance for your help :slight_smile: