Note: This overwrites the CI job setting Validate pull requests targeting the source branch for the final environment of your pipeline. For more information on that setting, you can visit this document here.
When setting up your Pipeline, you'll find a Run validations on final environment toggle in your pipeline settings. This gives you the option to decide how you want to run validation against your final environment's target org (typically Production).
For releases only
Validations against the final environment are triggered exclusively when a release branch is created or updated. Individual feature PRs moving through the pipeline do not get validated against the final environment; only the bundled release (grouping multiple features together) gets validated before it's promoted to Production.
For all pull requests
Validations against the final environment are triggered for every PR that targets the final environment's source branch, whether it's a single feature PR or a release. Every change gets individually validated against the target org (typically Production) before you're able to promote your changes.