The content_purge_boundary_date of each stream is the earliest date of content still available on the local disc of the Box:
For example, this value means that there is some content from 31 days ago that can still be retrieved from Box:
content_purge_boundary_date:7/15/2019 12:01:12 PM (31.3d ago)
The primary reason to know the content_purge_boundary_date is to verify that the the Box's disk capacity is sufficient for Lazy Indexing and Query Match Indexing. For example, if you're uploading Events automatically only when there is a human or vehicle present during nighttime hours, and you still want to perform ad hoc forensic searches for other Events during the daytime, then the content_purge_boundary_date tells you how long you have to realize the need to retrieve that daytime video that's not yet in the cloud.
Purged content appears in your real time feed like: