Further down the Product Backlog, it’s fine for items to be fairly fuzzy. It’s also fine for items further down the backlog to be whole projects – large, high-level items that are not so much User Stories but more like Epics!
As an item nears development, the item should be broken down further. And as it nears development, the item on the backlog should be defined in sufficient detail that the team can reasonably estimate its size and break it into tasks.
Until that time, however, it’s just really a placeholder. A reminder for prioritisation and high-level estimating. That’s all.
For some people, particularly those used to a more traditional project approach, used to detailed specifications up-front, this can potentially feel very uncomfortable. It shouldn’t.
The logic here is simple. There is little point defining a feature (or set of features) in detail if it may never reach the top of the priorities. The other aspect of this logic is that you tend to know more about your requirements, constraints, etc as time goes by.
And things change. People come and go. Sometimes the team has changed significantly since the original requirements emerged, so information can be lost if it is captured too early.
Therefore it makes business sense to defer details until they are needed.