What does this mean?
Responded not @ mentioned percentage tells you the percent of replies where the user was not @mentioned on the initiating thread.
This percentage is the proportion of responses where a Viva Engage user read a discussion and participated without being prompted in the thread. Comparing this to email, each time you craft a message you have to put someone in the "to" line meaning that you can accidentally exclude people who could add value to the discussion.
Measure of ROI
A major benefit of working out loud in Viva Engage is that users can organically participate in discussions. This ultimately saves email traffic as people work out lout together. The responded not @ mentioned % measures the value you are getting from having that discussion in Viva Engage.