What is Highlight Compression?
You can find Highlight compression parameter in:
Render Setup > Scene > Camera/Exposure/Tone Mapping
The Tone Mapping rollout in a Corona Camera
Render Settings > Corona > Camera/Postprocessing
Corona Camera tab > Tag Properties in a Corona Camera
Corona Postprocess dialog opened from the Picture Viewer
Post tab in the Corona VFB
Increasing it reduces the effect of burning in brightest parts of your image - this is useful for example if you want objects behind the window to be visible on a sunny day. There is no "correct value" for highlight compression as it has purely artistic value. Usually there is no need to increase it above 2-5 but in some cases you may even want to increase it to very high values like 50 or more.
Note: use default highlight compression value of 1 when you are saving render elements for later composing or saving 32-bit files for post production.
Highlight compression = 1 (default value)
There are very bright areas caused by direct light and refractions.
Highlight compression = 2
More detail is recovered from brightest areas.
Highlight compression = 3
Brightest areas are becoming "flat".
Highlight compression = 100
Image lacks contrast as bright areas are becoming less pronounced.
Highlight compression = 0,5
There is a lot of burning in brightest areas of the image.