Why, when measuring the quality parameters of Ethernet, the channel load is taken no more than 80%?

Question

Computer networks

Usually, when measuring the quality indicators of Ethernet (round-trip delay, packet loss, jitter), providers, and after them, customers, check them as accepted when the channel is 80% loaded .
I looked at RFC 2544 , ITU-T Y.1564 - didn't find anything about it.
So the question is - where did the figure of 80% come from ?

Answer 1

20% - service traffic, OS reservation for your needs, shaping thresholds, QoS features and a bunch of other data, rather than calculating everything and explaining everything to the client every time, it's customary to give 20% for everything secondary.

Answer 2

package transport overhead