New to Portfolio so probably missing something ....
Basing plans on epics, sizing and planning in story points.
Epics close to the top of the backlog are fully broken into stories and all stories are sized. Good so far ....
As you go down the backlog you reach EPics where the stories may not all yet be identified and may not yet be sized.... towards the bottom Epics may not have any stories identified yet.
How does Portfolio determine how many to allocate to each sprint in the plan? and what capacity its consuming etc?
In portfolio for an issue to show up in the timeline the issues needs to have some form of a unit of weight which needs to exist, primarily you have issues that are estimated and they are being scheduled by the automatic scheduling mechanism in Portfolio and showing up on the timeline in the scope. This is great for the issues that you have a clear a set estimate.
However for the other issues that are the unestimated options, to set a weight that can be recognized by the schedule you can use either a sprint, a release, or target dates set on the issues to give a time value that can be recognized as a weighted item so the timeline can populate the issue in a suspected range.