The way filters (groups) work is that they let you read or post to a sub-set of your access list or subscriptions list.
So, if I have a reading filter, it will let me see only a portion of the number of accounts I'm subscribed to -- maybe I'm subscribed to a community that posts 500 posts a day, and if I read my whole reading page, the community will drown out any other posts that people I'm subscribed to make. So, I create a reading filter that doesn't include that community, but does include everyone else, and I read that filter regularly and then go read the community itself when I have time to catch up.
Posting filters (access filters) are for if you want to make a post that's only visible by a few people on your list. Say I have my mother on my access list, but I want to bitch about her -- I'd make a filter that didn't include her and post to that filter. Or if I know that half my readers aren't interested in my locked posts about a thing, I'd make a filter that only includes people who want to read about the thing and post all Thing-related posts to that filter. Or, some people (myself include) have a very small filter of people who read their fiction that they're writing while it's still being written, to offer improvements and stuff. That kind of thing.
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So, if I have a reading filter, it will let me see only a portion of the number of accounts I'm subscribed to -- maybe I'm subscribed to a community that posts 500 posts a day, and if I read my whole reading page, the community will drown out any other posts that people I'm subscribed to make. So, I create a reading filter that doesn't include that community, but does include everyone else, and I read that filter regularly and then go read the community itself when I have time to catch up.
Posting filters (access filters) are for if you want to make a post that's only visible by a few people on your list. Say I have my mother on my access list, but I want to bitch about her -- I'd make a filter that didn't include her and post to that filter. Or if I know that half my readers aren't interested in my locked posts about a thing, I'd make a filter that only includes people who want to read about the thing and post all Thing-related posts to that filter. Or, some people (myself include) have a very small filter of people who read their fiction that they're writing while it's still being written, to offer improvements and stuff. That kind of thing.