A mapping is what Quick Connect uses to determine which object on a source connection links up to which object on a target connection. A mapping is used to maintain a 1:1 (one to one) relationship between a source and target connection and avoid the creation of duplicate objects.
When configuring a mapping between a source and target connection, you need to choose what object type is being mapped on each side. It's common that the object type is the same on both the source and the target connection in a mapping, however, the object type can be different on the source from the target. For example, you can map User objects on one connection to Contact objects on another connection.
Once a mapping is created, a mapping rule needs to be configured within the mapping. The mapping rule determines the unique condition that is met to match between the source and target object. For example, configuring a mapping rule between User objects on the source and User objects on the target, if using Microsoft Active Directory, you can use sAMAccountName on both sides. The sAMAccountName will always be unique in Microsoft Active Directory, so this is an ideal attribute to use when creating a mapping rule.
When a mapping is created between a source and target connection, it is a 2-way mapping. If you created the mapping while navigating into the source connection mapping, you can access the same mapping configuration by navigating into the target connection mapping.