Excerpt (sf) The full set of Apex tests on the task branch is automatically validated (checkOnly) against the packaging org or production org when the pull request is created. (TBD)
Note: The On a non-SF system, the equivalent might would be trying to run a local mock merge, compiling the source code, running the tests, and discarding the merge, build, and test deployment of the task branch to the staging or production environment, where the deployment is rolled back before it has any affect.
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