The Apple documentation says the following in the overview of the NewsstandKit object NKLibrary:
The downloadingAssets array holds any downloading assets for any issues. (A downloading asset is represented by a NKAssetDownload object.) A newsstand content application should, when it launches, iterate through this array and call downloadWithDelegate: on each item to have the downloaded assets completely processed.
I am using deductive reasoning to assume that once a NKIssue has had it's downloadWithDelegate: method called it keeps the status NKIssueContentStatusDownloading until it has been removed from the NKLibrary or downloaded.
Also, if a download has been interrupted and needs to be resumed, you would loop through all the NKIssues in NKLibrary's downloadingAssets property to re-call it's downloadWithDelegate to resume it. However, who knows when the downloadingAssets is cleared or reset. I don't even know where we would find that out.
That's the best answer I could find.