bmw battery and little gray hose

["FS" <freestat@xxxxxxxxxxx>]

OK, so I've removed the 'BMW' battery (after disconnecting the + and -) from
my '98 E36 M3 and I notice ~1/4" thick x ~6" long gray rubber tubing (marked
PP?) hanging from the right side of the battery.

So... can anyone tell me:

a) where this little hose gets plugged back into (when reinstalling battery)
and

b) its purpose?

Thanks!

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