Multiple destination publishing means that the transport service can publish to multiple deployer destinations (if you have more than one deployer).
This can be done by setting the strategy for the DeployerCapability which is registered in the discovery service:
(point 5 and 6)
DEFAULT strategy means that there will be only one deployer (only one url is registered in discovery-service).
MIRROR strategy means that is more than one deployer (multiple urls are registered in discovery-service) and transport service will send the package to all of the deployer urls register in discovery service.
For multiple destination publishing in Web 8.1, see below KB articleIn Web 8.1, is it possible to configure multiple content delivery environments for a single purpose?