Hmm, you may use 2015, 2018 or 2020 exe clients for your server but most of the servers today uses 2018 or 2020 clients because it is more latest and updated.
It will take time for me to explain about PRE-RE or RE. So better just read this on the RMS Forums.
- https://forum.ratemyserver.net/general-discussion/difference-between-pre-renewal-and-renewal-ro/