(as pictured below?) I’m sure this has been covered somewhere and their functionality is for the most part discernible via trial-and-error, but I was surprised I couldn’t find/figure it out just by “sight” or via some obvious documentation:
My “guess” is the icon at left mutes/plays the VST instrument in question you’ve loaded; the one in the middle apparently loads the instrument (when clicked and it’s highlighted?, though I’m not clear the relationship between this functionality and the actual look of the icon - sorry if this is obvious); and the right one allows you to assign Expression or Percussion Maps to specific channels within the VST in question (but thanks for any further clarification about their actual names and “official” functions!)
An obvious suggestion (if it hasn’t already been done and I simply haven’t “enabled it”, etc.): why not have some text that appears when you hover the mouse over each button, to better explain their names/functions?