From the PICA FAQ's - Question 21 regarding How Capacity Size is Determined:
“Capacity” means the total number of rounds that can be used of the maximum identified chambered sized round.
If the shotgun’s fixed magazine tube will accept more than 5-rounds of the maximum identified chambered size round, it is regulated by PICA and subject to the endorsement affidavit provisions.
Extension tubes are neither fixed nor detachable magazines, however, they are considered an "assault weapon" attachment subject to the endorsement affidavit requirement if used for something other than hunting.
If the shotgun’s fixed magazine tube will not accept more than 5 rounds of the maximum identified chambered size round, it is not regulated by PICA unless it has one of the features otherwise regulated – for example a pistol grip or thumbhole stock.
Section 1.10 of PICA does not regulate shotguns but rather regulates large capacity ammunition feeding devices. With respect to magazines, belts, drums, feed strips and other similar devices for shotguns, if it accepts more than 10 rounds of any type of ammunition it is regulated by PICA; however, the endorsement affidavit provisions do not apply to large capacity ammunition feeding devices.
There is a specific exemption for tubular devices designed to accept, and capable of operating only with, .22 caliber rimfire ammunition.