What do you mean it is hard to understand?????:
"20-ampere circuits - 16 AWG and larger" cords are allowed.
There is also 240.5(B)(1) and 410.30(C)(1). If you have a listed fixture or appliance with an attached cord that fits into a 15 or 20 amp 120 volt receptacle, then the cord size will be dandy.
If you have some non-listed fixture or appliance, then one had better take a closer look.
