In Europe the laws and regulations in regards to motorcycle use on the road are becoming more stringent as months and years move forward.
E Marking is a regulation under which some countries dictate for bike or component to be road legal and pass the yearly MOT or equivalent tests.
Many Tail light, Indicators, Exhausts for example must be E Marked for Road use to be fitted and run. Road homologation dictates that the product is compliant for use in the EU and can be safely be used. In the Exhaust example this would mean that the Sound and emissions comply with the restriction applied to them by EU regulations.
Many countries dont have such regulations and items that are categorized under "RACE USE ONLY" can be used on the road.