@BzzBzz to me a non original manga is simply a manga base off a novel, similar to many movies. To me a promo is something designed to give you taste but not the complete meal.
I've made a similar suggestion and I was talking about translations I saw where there is only 1 chapter or maybe a couple but it is not the full story. Some have even mentioned the novel in the chapter. One that I think of as a promo is about a woman in a fantasy world goes to prison but has brought along all the supplies to live a life of luxury. They did two early chapters and then for the third chapter jumped way into the story, almost to the end.
This one is another example, nowhere on the title page, that I can see, is there a reference to it being a promo. It is marked as a oneshot and marked as completed. It is not completed, it's not even hardly started. The group page of the chapter even states it is a promo. https://mangadex.org/title/50535/i-don-t-want-to-be-a-magpie-bridge
I think that is what
@yourisekaiwitchess means. For me I like reading oneshots but some are promos and not easily filtered out if they don't have a tag. Some do not even have a mention of promo in the description or chapter title so you don't know until you start reading and waste your time. Even when I'm not searching for oneshots I would still like to know if it is a promo or not.
Edit to fix link