Transport is a framework for transferring data between Drupal sites.
The concept of a Drupal entity is backported from Drupal 7, and any kind of data such as nodes, users, terms, comments, and files are considered as 'Drupal entities'. Transport can work between different versions of Drupal, and hence can be used for data migration from older versions of a site with UpRPC module.
Entities can be extensively altered during the transport process, which allows for cases such as moving data between fields, preserving data on the local copy of the entity, and so on.
This documentation section contains examples of how to modify transport data.