Yes it does matter.
Each book purchase/loan is associated to just one Adobe ID; when it first opened it is locked to the ID in use at the time.
It can then be used on other computers/devices registered to that ID.
To change the ID on ADE: use ctrl-shift-D to unregister it (cmd-shift-D on Mac); then register again (Help/Authorize Computer)
To change the ID on a device connected to ADE: use ctrl-shift-E to ADE while device connected to unregister it (cmd-shift-E on Mac); then register again (Help/Authorize Device)
Unfortunately, the Adobe team did not go on user interface design 101 and learn that features such as this should be discoverable.