Since the term RSO is a generic term often used to describe highly concentrated, crude cannabis oil often marketed to cancer patients, it comes with plenty of hype and stigma. To be more accurate, we prefer to use the term FECO (full extract cannabis oil) RSO is often used interchangeably with FECO , but these are not necessarily the same products.
FECO and RSO are essentially the same product, a crude cannabis extract often sold in a syringe. However the primary difference between the two is that FECO is almost always ethanol, whereas RSO is a more generic term and could be made using isopropyl alcohol, ethanol, hydrocarbon solvents and even CO2. In the end, there is no clear definition for these crude extracts, and each solvent will have its own unique properties that extract certain chemicals from the plant better than others.
FECO is generally considered safer than RSO because ethanol is generally safer for consumption than isopropyl alcohol or naphtha, both known for being flammable and toxic (though naphtha is worse in both regards)