“A virtual environment is created on top of an existing Python installation, known as the virtual environment’s “base” Python, and may optionally be isolated from the packages in the base environment, so only those explicitly installed in the virtual environment are available.”
However, PEP 405 – Python Virtual Environments | peps.python.org says “By default, a virtual environment is entirely isolated from the system-level site-packages directories.”
The phrase “may be optionally isolated” can mislead beginners and create confusion, especially for people trying to understand how environments work. The environment is isolated by default, and no options are needed to disable isolation. A better phrasing could be: “may optionally have isolation disabled” or “may optionally be allowed access to global packages.”