Hi. For pickup arrangements like the attached link, is the middle pickup used for MIDI or is it for hum cancelling?

http://www.alembicguitars.net/prod/seriesii.html

The middle "pickup" on a Series bass is to cancel hum from the two single coil pickups it isn't used to amplify the strings. The following FAQ contains directions as to how it works.

http://club.alembicguitars.net/index.php?topic=269.0

Thank you. I wasn't sure of its purpose.

Some non series basses could have a dummy hum cancelling pickup as a visual thing. If a signature bass or a bass with Anniversary electronics is made with view to add series electronics in the future, a non connected hum cancelling third pickup is added to be later used when the conversion to series I or II is done.
Here is an example of a non series alembic bass that is series prepped from new.
http://www.statii.com/forum/forum/viewthread.php?thread_id=6518&pid=55800