Everyone. Hashtags are just a means of adding a search term to your descriptions. For instance, if they were to do a challenge here regarding creations, they might ask you to use a hashtag like #CG4Challenge1 - that would denote Carl's Guide for Sims 4, Challenge #1. It would make it easy, though, for the challenge creator(s) to find the appropriate entries. But again, it's just a search term. Another example - I've added #dellena to many of my recent creations. Because people know me as Dellena here on the forums and then they don't necessarily need to know my origin ID.