The action flow rule has a tab for treatments where we can enable a channel and map the treatment for that channel for instance I am enabling Email channel and selecting an HTML treatment for it. Then in the flow configuration, I am adding a send Email shape and I can select any html treatment for my email.
My understanding was that the treatments configured in the treatment tab will only be available for selection within Send Email shape. Trying to understand the need to add a treatment in the treatment tab, I can understand the need to enable a channel for that offer but not sure about the use and linkage between the treatment template selected in this tab to the send email shape templates we select.
For Email, If you want the treatment to be determined by strategy then you need to define treatment in Treatment tab and select 'use associated treatment' offer flow Email shape. Otherwise if you don't want strategy to determine Treatment and want to send a pre-determined treatment then you need to select 'Specify treatment' radio button and select the specific treatment.