That is to say that the 20A circuit is in continuance throughout the circuit , however the receptacle itself (the contacts) are rated for 15A continuosly. A outlets are totally legal on 20A circuits. Because the NEC makes an exception specifically for that (National Electrical Code): enter image description here. The exception is by careful plan.
It would be stupid for electricians to have to carry two types of receptacles on their truck.
If the circuit shorts then the breaker will kick at 20A however.
This is legal and will work fine.
But, in the real world: - most electrical components are designed to handle roughly double their rating. A duplex receptacle is considered as multiple receptacles and is therefore permissible to use as the single, or one of several, multiple type receptacles on the circuit. Part of the UL listing for the amp receptacles is that they are capable of feeding through the amp circuit , the primary difference between . As the branch wiring and breaker is rated for 20A , those will be OK.