Button group
Joined buttons for related actions. The split-button (primary + caret) and the radio-style toggle group both live here.
Split button
Primary action on the left, secondary actions in a dropdown to the right.
Toggle group
Radio-style — exactly one button is "on" at a time. Used for view switchers, severity filters.
Do & don't
Do
Use a split button when 80 % of the time the user wants the primary action and 20 % needs an alternative.
Don't
Use a button group for unrelated actions. Joined visuals imply joined intent.
Do
Pick one canonical style per surface. Don’t mix segmented and outline-joined on the same screen.
Don't
Make every button in the group equal weight. The leftmost is the primary.