Principle of resume task

I want to know whether this plug-in supports resumable uploads? During use, I found that when I click to pause or the network is interrupted, I can select resume to resume the download. But if you kill the app and enter, the log print is "can't resume...".
I want to know what is the #principle of resume task? Will the plug-in temporarily save the downloaded file in the local database? Then download the merged file when you resume? Is the range field of http used?