At the UI's bottom you can see the timeline. Let's go through its associated elements one by one from left to right:
A) Lock/Unlock Button
When the timeline is locked then you will not observe any progress in time, and the simulation is “pinned” to the current frame. This means that the simulation is running, but frames are not incremented – the simulation is performed at a single frame, e.g. 0 or 50. As a side effect, animation data will not be evaluated. In order to unlock the timeline, stop the simulation with a click on “Simulate”, and click on the button again.
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B) Lock/Unlock Menu
Next to the - ”Lock Button” you can see a small triangle. A click on it reveals a context menu, giving you the possibility of specifying a frame when the simulation will be unlocked . You might argue that the timeline is locked, and frames are not counted, but in fact, (RealFlow counts the frames internally). Please take a look at → "Quick Start Workflows - Locked Timeline".
C) Simulation Start Frame/Playback Start Frame
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The elements of this area should be familiar to you, since you can find them in any 3D program, and electronic devices, such as media players of any kind. The elements are (from left to right):
[ I: playback buttons + explanation from confluence page ]
1. Go to first frame | 2. Go one frame back | 3.Play backward | 4. Current frame | 5. Play forward | 6. Go one frame ahead | 7.Go to last frame | 8. Loop | 9. Create OpenGL preview