How do i know what category i should be racing in w/kg ?

We’ve got a breakdown of groups in our events article, and we’ll be working on other in-game cues in the future. Stay tuned!

The linked article doesn’t answer his question, which is the same as mine.  Is the wattage listed based on FTP?  Or the wattage someone wants to train in?  Apart from races, won’t it change based on what pace someone wants to ride on a given day?

It is your FTP divided by your weight. 

That is the result of the official FTP test.