gtm global availability difference

hi, i am a newbie to GTM and am trying to understand some of the load balancing options available. I'd like to know if there is any difference between the Global Availability option at the Wide-IP level compared to the Global Availability option at the Pool level - is there any difference?

The manuals mention that the Global Availability option selects the Virtual Server in the order listed, however at the Wide-IP level, there are no VS to be configured....

thanks!