@@ -58,6 +58,27 @@ cargo install mdbook  # Installs mdbook locally
 mdbook serve  # Serves the docs locally
 ```
 
+## PR workflow runners
+
+The `PR` workflow uses the `ci-v3` RunsOn environment when the repository variable `SP1_CI_RUNS_ON_ENVIRONMENT` is empty.
+Set this variable to `legacy` to select the existing runners for new ordinary jobs.
+To select another RunsOn environment for ordinary PR, push, and merge-group jobs, set this variable to its environment name.
+Prepare that environment before you set the variable.
+The selected environment uses a `150gb:gp3:750mbps:4000iops` root volume.
+GPU jobs keep their instance type and AMI.
+
+The GitHub-hosted `Runner routing` job validates the selection before the nine RunsOn jobs can start.
+An invalid environment name or volume fails that check.
+The variable affects only this workflow.
+
+Manual runs use their `runs_on_environment` and `runs_on_volume` inputs independently of the repository variable.
+An empty manual environment input selects the legacy runners.
+
+Clear `SP1_CI_RUNS_ON_ENVIRONMENT` to restore the `ci-v3` default for new ordinary runs.
+This does not move jobs that already have runner labels or stop their instances.
+
+Run `ruby .github/scripts/check-runner-routing.rb` to check the default, legacy, configured, and manual routes locally.
+
 ## Publishing
 
 SP1 crates are hosted on [crates.io](https://crates.io/search?q=sp1). We use