eZ Publish Platform (5.x) was a transitional version of the eZ CMS, bridging the gap between the earlier generation called eZ Publish (sometimes referred to as legacy), and the current eZ Platform Enterprise Edition for Developers.
eZ Publish Platform introduced a new Symfony-based technology stack that could be run along the old (legacy) one. This fluid change allows eZ Publish users to migrate to eZ Platform Enterprise Edition for Developers in two steps, using the 5.x version as an intermediary stepping stone.
The migration process from eZ Publish Platform to eZ Platform Enterprise Edition for Developers is described in Upgrading from 5.4.x and 2014.11 to 16.xx.