Transmission rate is the transmission speed and is measured in bit-rates.
Transmission speeds can vary from 56Kbps modem speed to 3Mbps broadband speed and the quality of the transmission is equivalent to its speed.
Number one rule for selecting the correct transmission rate is ensuring that it is less or equal to the internet connection speed of your targeted audience. That is if your audience connects to the internet using 128Kbps speeds then the profile you choose must have a bit-rate equal or less than 128Kbps.
When you select a transmission rate you are actually selecting a 'Windows Media Encoder' because Windows Media Technologies allows for the use of the on-going record process for the transmission using the same bit-rate.
The Windows Media Encoder dialog presents you with additional information for every profile so that you can verify if that profile is perfect for your particular needs :