Those terms are often used very loosely by the listing agents. The listing that says "non-conforming 4-plex" or "non-conforming STR (short term rental)" may actually be an illegal use prohibited by the current zoning or land use code. Generally the term "non-conforming" or "grandfathered" means it was allowed (i.e. legal and conforming) at some prior time, but doesn't meet the current requirements for such same use. This could be for one of numerous reasons: lot size is too small, or the zoning has changed altogether, or it doesn't conform to the construction requirements for such use (think the large house with a single front door, but multiple "apartments" inside. Each municipality will also have their own specific requirements for what happens in the event of a casualty where all or a portion of the building is destroyed. Some will require that if greater than X% is damages (say 50%), you'll be required to come into full conformance with the building and zoning code. Others will say you have the right to build back 100% so long as you match exactly what was existing, but usually (actually almost always) according to the new building code. Determine whether your location is governed by the city or county and then search that place in Municode - I've included the link for Los Angeles County below (although you should call the city/county and confirm this is the correct governing municipality).
https://library.municode.com/c...
In hot markets, these differences usually don't matter as much as investors are happy to get their hands on just about anything, but in down markets it becomes a bigger deal, and this can also cause issues with getting insurance. Hope this helps some.