The original creators of the UTCI scale didn’t make a “slight heat stress” category as you point out in the scale you posted. So it is not something that is endorsed by the original creators of UTCI. However, they made a “slight cold stress” category, which makes things impractical when you want to have a clean 5-point, 7-point or 9-point scale for producing Ladybug graphics like this:
So, for practical purposes, I added in a “slight heat stress” category to make things balanced and I admittedly chose 28C somewhat arbitrarily by looking at the ratio between the slight cold stress and moderate cold stress categories (attempting to mirror it). I am re-thinking this for Honeybee[+] as I really prefer to build on the consensus of others and it seems truer to the original standard to just lump slight and moderate cold stress into one category if I want to make a clean 7-point scale. I also plan to expose options to get the original UTCI categories in the Honeybee API and I am thinking of making the limits of the categories set-able in HB[+] so that people can use their judgement if they want to. I have an initial draft of this on this branch:
I should also mention that there are other examples of practitioners adding their own categories on top of the original UTCI scale for practical reasons. For example, I know that people have referred to UTCI between 18C and 26C as “classic comfort” to denote conditions that should be comfortable when not walking or wearing a jacket. I imagine that the original UTCI creators would not endorse this as the entire UTCI scale is derived from models of people walking and changing their clothing so they would probably suggest making a new database of comfort simulations for such a purpose.
Needless to say, making a widely-usable system of comfort categories is not easy and I know that the PMV and Adaptive models have entire committees of people who regularly assess categories of acceptable comfort that get published in standards such as ASHRAE-55. I don’t know of such a regularly-meeting committee for UTCI but if anyone knows of one, please post here as I should definitely be following them.
Also, needless to say, if anyone has any thoughts on the HB+ implementation, please share them!