The Panasonic GH5S is a video-focused Micro Four Thirds camera built around what the company markets as a 10.2MP sensor. It's best understood as an even more video-centric variant of the GH5: it can shoot either DCI or UHD
4K footage natively (one capture pixel = one output pixel) at up to 60p.
Panasonic wasn't the first company to introduce high quality video to what was otherwise a still camera, but with its GH series it has been constantly expanding the range of professional video features appearing in consumer stills/video cameras. The GH5S takes this logic one step further, by lowering the sensor resolution and omitting image stabilization to make a more single-minded video tool, rather than an hybrid intended to be similarly capable at both disciplines.
The ability to shoot DCI 4K at up to 60p with no crop is the most obvious distinction between this and the standard GH5, but the differences run deeper:
Oversized 'Multi Aspect' sensor with dual gain design
10.2MP maximum usable area from at around 12.5MP total
DCI or UHD 4K at up to 60p
10-bit 4:2:2 internal capture at up to 30p
8-bit 4:2:0 internal 60p or 10-bit 4:2:2 output over HDMI
1080 footage at up to 240p (with additional crop above 200p)
Hybrid Log Gamma mode
ISO 160 - 51,200 (80 - 204,800 extended)
AF rated down to –5EV (with F2 lens)
3.68M-dot (1280 x 960 pixel) OLED viewfinder with 0.76x magnification
1.62M-dot (900 x 600 pixel) fully articulated LCD
14-bit Raw stills
11 fps (7 with AFC) or 1 fps faster in 12-bit mode
USB 3.1 with Type C connector
As well as the ability to shoot DCI 4K at higher frame rates, Panasonic also claims the GH5S's larger pixels and 'Dual Native ISO' sensor will mean it shoots significantly better footage in low light.