Patch Test Realm (PTR) is a server that Blizzard allows players access to to test future patches. They are online temporarily and shut down when the patch being tested has gone live on the regular servers. This is only available to PC BattleNet users.
The first PTR will begin with Season 4 content that promises to see "sweeping changes" to Diablo 4 and how affixes, item stats, the Codex of Power, and new in-game systems will change for both Seasonal and Eternal Realms.[1]
During the PTR, we’ll listen closely to your feedback to make adjustments before the Season arrives for all. [2]