So, it's early doays for me with Olympus Pro Capture on the OM-D E-M1 Mark iii. Pro Capture buffers frames before as you half press and before you full press the shutter release button
Pro Capture Low carries out autofocus on all frames. Pro Capture High only autofocusses on the first frame
Pro Capture Low is disabled (menu is dimmed) when you are not using an Olympus Pro lens.