The majority of CBD E-Liquids are made using CBD Isolate and contain no other cannabinoids other than CBD. This is the most economic way to create a CBD E-Liquid.
Full Spectrum CBD E-Liquids is extracted gently with CO2 from the whole hemp plant, and will contain small legal levels (>0.2%) of THC as well as other other cannabinoids.
Broad spectrum E-Liquids are made with extracts that have been distilled. This refinement is one process further than the full spectrum and therefore removes further more undesirable content along with all the THC.
A thing to consider is that sometimes during the filtering and refining process to reach a broad spectrum THC free extract some of the minor terpenes and cannabinoids can also be lost.