This section discusses employment-based immigrant visas, not non-immigrant visas. Immigrant visas are visas that are obtained when you wish to get a green card in the US (lawful permanent resident status). An immigrant visa is for a person who wants to work at a company not for a short period of time but wants to permanently remain in the US. These are I-140 petitions.
An approved I-140 petition may be used as grounds for adjustment of status (obtaining a green card) with a company other than the petitioning company. If the company that petitioned for you no longer exists, no longer employs you, or no longer wishes to help you get your green card you may be able to port your approved I-140 to a new company and avoid having to start the process all over again.
USCIS has issued a Policy Memo regarding portability in the context of I-140's. The memo provides guidance do adjudicators on how to determine whether a new job is in the same or similar position as required under INA §204(j).