Hovering your mouse over each of those buttons will reveal a tooltip that indicates each buttons purpose.
The 4 on the left are the Standard View Skins PlayerBar Previous, Play/Pause, Stop and Next Buttons.
NOTE - Whilst the MC PlayerBar Previous and Next buttons can be used to adjust the Tracks Playback Position Backwards (Previous Button) or Forwards (Next Button) on the Popup the Previous Button will set the Previous Track Playing and the Next Button will set the Next Track playing.
The Tooltips for the Previous, Play (when visible) and Next Buttons display the buttons purpose and the same information the CoverArt Tooltip for that Track would display.
The ones in the middle:-
For the ET Ultra Control and ET Ultra Control 3-Tracks they are customised buttons, that will be the same for every Standard View Skin, the 1st reduces the MC Volume by 5%, the 2nd Mutes/Un-Mutes MC, the 3rd increases the MC Volume by 5%.
For the ET Ultra Slider and ET Ultra 3-Tracks Slider there is a customised Mute/Un-Mute MC Button alongside a representation of the Volume Slider from the Current Standard View Skin.
The 3 on the right:-
The 1st one is the Standard View Skins Topbar "Minimise" Button and is used to Hide/Minimise the Main MC Window.
The 2nd one is a customised Playlist button, it will be the same for every Standard View Skin, clicking on it will reveal a list of the tracks currently in Playing Now; clicking on any of the track in that list will result in the That Track being Played.
The 3rd one is the Standard View Skins Topbar "Maximise" Button and is used to Show/Maximise the Main MC Window.
N.B. The Minimize and Maximise Buttons displayed in the ET Ultra Control, ET Ultra Slider, ET Ultra Control 3-Tracks, and ET Ultra Slider 3-Tracks window will be identical to those displayed in the Topbar of the MC Playing Now window for the Platform MC is running on.