Thank you for asking this. I meant to do a post on submissions for a while now and this is a good prompt for it. I don't know if you want to call it formal or informal, but as a community lead I have developed my own little formula to which I adhere when deciding what to repost.
1. First preference goes to the posts by Ello folks who have never submitted anything and are new to Ello or to writing, have few followers, few posts, so I repost them as a signal boost to grow the community.
2. Second preference goes to relevant pieces of writing that are in tune with what's happening in the writing world. You have seen a bunch of NaNoWriMo reposts in November and you see some coffee posts in the mornings and you see some quotes and writing prompts and detailed analysis of this writing problem or other. News about contests and grants and collaborations and whatnot go here too.
3. Third preference goes to questions, concerns, calls for help (if you're blocked), just personal observations on why one writers, as such posts inspire others to write and to share their process.
4. Fourth preference goes to posts by members who have been reposted several times already. I mark the submissions with the little heart and save them for later, and when there is a window, I scroll to the oldest submission and either repost it or go to the member's profile and see if there is anything new I can repost.
5. Fifth preference goes to new releases, the book you just published (when you post a cover picture with a little blurb), a poem you published, anything. We want to celebrate with you!
6. ON TOP OF THIS ALL I try to vary the posts, so if you scroll down you see fiction pieces, non-fiction pieces, poetry, articles, images, quotes, etc. It's a bit of a balancing act, really, to keep the audience entertained and not bored.
7. AND ON TOP OF THAT any exceptional pieces of writing take first preference and I repost them regardless of whether someone has been reposted already or not.
Now to the things that will not get your post reposted.
1. If you keep submitting same content (say, excerpts from the same draft you're working on) over and over and over again, I simply glaze over it and wait for you to post something new.
2. If you post links to outside content sources, I won't repost it. Readers don't like clicking on links, so at least include a summary or an intro and THEN a link to your piece.
3. If you submit something that is hard to read (grammatical mistakes, etc.) or is in a different language. I usually bump it up to the @ellolanguages community, or make sure you include your piece in both English and the other language.
4. If your post is much longer than 2 pages (2 screen pages) and people have to scroll forever to read it.
5. If your post if very brief, one line or so. Unless it's absolutely stunning, it probably won't get reposted.
6. If you tag a ton of communities in your post, like poetry and photos and lots of others. (Sometimes I make exceptions.)
7. If for one reason or another I decide that your post will not benefit the community in any way, and I can't give you specific reasons for this. I just go by my gut.
So, having said all this, IF YOU HAVEN'T BEEN REPOSTED PLEASE KEEP SUBMITTING FRESH WRITING. There will come a moment when a gap might present itself for your post to be reposted.
Oh, and one more thing. When this community started, I was reposting up to 6 pieces of writing a day. It was too much. As community leads we discussed the frequency of posts and the decision was made not to post more than 5 a day and not more than 2 an hour. I have reduced the reposts to 5 a day. It was still too much. There is no automated system that does it, I do all reposts by hand, and unless I reposted 2 posts at a time, I couldn't do it. So I reduced it to 4 posts, then to 3 a day. One in the morning, one in the middle of the day, one at night. Recently I noticed that when I reduced the reposts the number of members started growing faster. I have also noticed that the posts at night don't get much attention, so I stopped. Now I repost 2 times a day, in the morning and some time in the afternoon.
I hope this helps. Please keep submitting new writing. And please let me know if you have any questions or suggestions or comments on how to make our Ello writing community better. Much love. XOXO