I-797B Substitution of Beneficiaries

I got my I797 Approved and have following details on it:
I-797B, Notice of Action
If any of the worker(s) included in this petition do not actually enter the United States, and substitutions of different workers are not made, the petitioner must notify this office [USCIS] so the allocated nonimmigrant visa numbers can be re-used.

Could anybody please explain what does it mean? If I leave my employer/petitioner, would the petitioner be able to transfer the visa to other alien?
If the petitioner cancels my visa, can I still get the visa ported to other employer?

I have H1b stamped also but have not traveled on this H1B. Would the petitioner be able to transfer the visa?