What exactly is throughput?

A user asked about the difference between data rate and throughput, and an answer was:

Bit rate refers to the number of bits per second that the physical medium can transfer. Throughput usually means how many bytes per second of useful data can be transferred through the link.

Does "useful data" mean payload?

My textbook says:

If you got a hub with three interfaces, each 100 Mbps of capacity, and two of them are trying to transmit 100 Mbps to the third, then the third would receive 200 Mbps, which is not possible. So the two hosts connected to the first two interfaces will have an effective data rate of 50 Mbps each.

For me, these 50 Mbps are not "useful" data. They're just data. The amount of data that is actually being transmitted.

Is my textbook wrong? If so, which concept would you use to describe these 50 Mbps?

Is there a difference between throughput and effective data rate? This article differentiates between the two. Though I've seen many times throughput = effective data rate.

Another definition of throughput:

Network throughput is the rate of successful message delivery over a communication channel.

Does that mean that if each packet has to be transmitted twice because of some failure, then the throughput would decrease?

Which concept would be "rate of (either successful or unsuccessful) message delivery over a communication channel"?