Trimming for 100-nit Rec709
If you have received the above warning in your Dolby Vision XML check, it is a warning that your HDR project was Trimmed for a Target Display above the 100 nit Rec709 Target- presumably 600 nits or 1000 nits.
What this means is, you will not able to derive a 100 nit Rec709 Target Trim from the Metadata, and consequently, any Display below the Threshold that you have trimmed for will not be valid.
If the studio or content provider requires this minimum 100 nit Target, you will then have to go back to the color corrector and Trim for this target again.
Trimming for multiple Targets.
Dolby Best Practices encourages you do the 100 nit Trim first. There are a few reasons and advantages to taking this approach.
By trimming for 100 nits, you are assuring that the mapped Metadata will match and be correct for all the Targeted Displays up and down the ecosystem.
If you are required to produce a Trim value at 600 nits, by trimming for 100 nits first, you will be starting from a better place when you do this second Trim pass, as the mapping algorithm will have put you in a much better starting point, and that secondary 600 nit Trim will be for the most part already very good leaving you with much less secondary Trimming for the 600 nit Target.