Maya 2018.5 is not a separate purchase; it is the update to Autodesk Maya 2018. Autodesk moved to a subscription-only model, releasing incremental "Extensions" throughout the year.
Maya 2018 expanded the toolset, moving it from a niche tool to a core part of the workflow for motion graphics. autodesk maya 20185
Perhaps the most significant update in Maya 2018 was the integration of . Maya 2018
: 64-bit Intel or AMD multi-core processor with SSE4.2 support. autodesk maya 20185