An acsm file is a text "ticket" to redeem an epub with DRM. ADE is the official method, but downloads an epub encumbered with DRM. An acsm file from Kobo expires in about 6 hours and then you need a fresh download of the acsm if you didn't redeem the ebook. Once an acsm is used with a particular Adobe account a fresh download of the same acsm (say you accidentally deleted the epub) needs the same Adobe account. An acsm for a different book can use a different account.
The plug-ins on Calibre automate the process and can use your ADE/Adobe account or a different scheme, but not for the same ebook.
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