Some of the UWP codes are NOT trade codes.
Only Agricultural/Non Ag, Industrial/Non Industrial and Rich/Poor are true trade classifications. Hi/Lo Population is sort of a grey area.
The other classifications, Vacuum, Desert, Fluid etc are WORLD classifications, but don't really deal directly with Trade as such. Sure, Vacc Suits will sell better on Vacuum worlds.
While each world classification might give you a logical background skill, calling them Trade Classifications would be wrong.
Really, the original tables gave you 3 categories of Home World Classifications:
World Classifications (vacuum, Fluid, Ice Capped, etc)
Trade Classifications (rich/poor, Ni, I etc)
Tech Level Classification (Pre-Stellar, etc)
Each type of classification can give you a possible background skill.
Incidently, Barren was a world classification indicating that the population was ZERO. How can someone be from a world with a ZERO population? Even if you said that was really 0-9 people. How many people within a star-faring culture are going to be from one of these places that you need to have an entire background skill category for it?