503 Service Unavailable
Meaning
The server is currently unable to handle the request due to temporary overload or maintenance. This is usually a temporary state.
When to Use
When the server is down for maintenance or is overloaded and cannot handle requests at the moment. It indicates that the condition is likely temporary and the same request might succeed at a later time.
Example
If a website is undergoing scheduled maintenance, it may return 503 Service Unavailable with a message like 'We'll be back soon' and possibly a Retry-After header to tell clients when to try again.
When Not to Use
Do not use for permanent failures or unrecoverable server errors (500 series). 503 should not be used if the service is operational but the request itself is faulty (that would be a 4xx). It's specifically for temporary unavailability of the service itself.
Source
RFC 9110