OK, I was wrong- it is if you have over 100 posts:
http://www.dakkadakka.com/dakkaforum/posts/list/362462.page#2697167
yakface wrote:It means that since you have 100 posts, you are now a 'trusted user' (in that you're not just a spambot), so you are now able to 'tag' images if you want with tags that will give those images search tags so when people search for tags, they will find that image. You can enter as many (or as few) tags for an image you're rating...just make sure you separate each tag with a comma.
The system will automatically ask you now if you'd like to tag any image you're rating that has less than 4 tags on it. You can always choose to ignore the pop-up and continue on (you don't have to tag an image if you don't want), and if the pop-up bothers you, you can turn it off in your user profile.
And pretty much everything I just wrote here is included on the pop-up itself if you read it