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I posted yesterday but was referring to it as filtering by channel. It doesn't exist now, but since it is in beta I am hoping they will add something. The channels are not really channels here... they are just tags people choose when they post.

All you need to do is use a tag, and if the tag doesnt already exist then it will create it for you. Tags are pretty much the same thing as Subreddits. You can also use hashtags in posts to post in multiple "subreddits"!!