During the period in which the shop has the Offline maintenance status, any dynamically generated web page will display the maintenance messages set in Setup - Maintenance shop.
Moreover, an additional header that indicates the maintenance status 503 Service Temporarily Unavailable will be sent to browsers and search engines.
The HTTP 503 Error header signals browsers and search engines that at the time the server cannot process the request due to a temporary server overload or maintenance of the server. The implication is that this is a temporary condition which will be alleviated after some delay.
Based on the maintenance status HTTP 503 Service Temporarily Unavailable, search engines will not index any page during maintenance.
Listed in: Shop settings