Infrastructure Epics are larger stories. In the Storymap Tree you can have as many levels of epics as you like to describe the project. Once you reach a level which cannot be broken down into smaller parts, you create stories.
Infrastructure Epics can be planned to releases, but cannot be put in sprint – only stories can go into sprints.
The state of the Infrastructure Epic will always follow the state of the stories/epics below it.
See more details on Epics and Stories and how they are represented in Agemba here.