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.
What is on the PTR
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]
Because PTRs run for a short period you are often supplied with pre-made characters to give you a head start enabling you to begin testing content.