S-Log is a gamma curve with a wide dynamic range optimized under the assumption that grading will be performed in the post-production process. S-Log3 allows for better reproduction of gradation characteristics in shadows and the mid-tone range than S-Log2. It has characteristics closer to those of scanned film.
S-Log3 is recommended in general. If S-Log3 does not offer sufficient gradation in the mid- to high-luminance range, try S-Log2.

When using S-Log, we recommend checking the image quality including grading in advance by test-shooting.
When grading videos recorded with S-Log, technical knowledge is required.

For details on S-Log, refer to the page below.
Help Guide for Creators
http://helpguide.sony.net/di/pp/v1/en/contents/TP0000909108.html

NOTE: Help Guide for Creators is a Help Guide for more creative image production using a camera manufactured by Sony.