The Series 66 merges the ground covered by series-63 and series-65, but it assumes the product knowledge tested in the series-7 and can only be used together with it. Someone who fails or lacks the Series 7 must take the Series 65 route instead.
It suits the dual-registered path, where an individual is both a registered-representative of a broker-dealer and an investment-adviser-representative of an affiliated adviser, a common structure at large wirehouses.
As with every exam in this family, the licence governs what you may sell and to whom, not how you may trade your own account. Personal trading is governed separately by firm policy and by rules on material-non-public-information.
Related: series-7, series-63, series-65, investment-adviser-representative, registered-representative