Well a contract can be whatever you agree. Unless any of us are party to the detail of the contract then we won’t know. It could however be one of the following options.
OPTION 1~ only the stations the handling agent operates at are contracted and the airline has to find another alternative for the other stations.
OPTION 2 ~ a contract is entered into for “all stations” and where the handler is not present the handler has to contract the services of another handling agent through a sub contract agreement
In my experience option 2 is very rare as there are issues around article 8.5 on the SGHA which covers liability and the airline does not have the same control over the service provided as they are not contracted with the agent delivering the service.