Originally Posted by Roelof

The 5v specification is very old fashion which comes from the 5v components of the receivers (74xx TTL logics) like the old Robbe Economic. Later receivers were using 40xx CMOS logic IC's which can handle with easy up to 7v.

The 6v specification was based on the 5 cell NiCad/NiMh receiver packs but those batteries in full fast charge can give up to 7.5v. Most servo's were using the ZN409 servo controler which was able to work up to 6.5v

The 7,2v specification for ESC's is based on 6 cell NiCad/NiMh batteries but in full fast charge they can have a voltage beyond 8v.

The 6.6v is the nominal voltage specification of LiFe but comes from the charger with 7.2v

And yes, the 7.4 specification is (again) the nominal voltage specification of a LiPo/Li-Ion battery.

There is a lot of confusion about 7.4v, if you do not agree my standpoint then you must explain where the 7.4v comes from. To set more strength to my standpoint is that 7.4v is also a HV specification and HV is also a name for 2S LiPo. And also explained LiPo 2S Battery 101: All About LiPo 2S Batteries (batteryequivalents.com)