The terminology is a bit inconsistent. I ran into that problem while trying to tidy up Sarna articles. In particular, there seem to be three different definitions of what constitutes a Docking Collar:
Docking Collar:
(I) Basically anything that will allow two spacecraft to dock with each other and transfer stuff and people. Intrinsically built into every spacecraft, at no specific extra cost during construction.
(II) In a somewhat more restrictive sense, the defining element of a DropShip - a docking collar with KF Boom extension that will permit a JumpShip Hardpoint/DropShip Collar coupling.
(III) Finally, also often (and misleadingly) used synonymous with "Hardpoint".
Hardpoint:
A docking collar with KF Boom on a JumpShip (or WarShip), ready to dock DropShips and carry them along in a jump. Often (misleadingly) called "Docking Collar" or "JumpShip Collar".
DropShuttle Bay:
A primitive precursor to the Hardpoint, widely used on primitive spacecraft back when there was no distinction between JumpShips and WarShips. Amounts to a super-large small craft bay and will carry DropShuttles (of up to 5,000 tons) through a jump. Details unexplained so far but implication is that DropShuttles/DropShuttle Bays, unlike small craft bays, had some sort of KF coupling which is incompatible with modern DropShips.
Small Craft Bay:
Can dock Small Craft of up to 200 tons and will carry them through a jump within the JumpShip's (or DropShip's) jump bubble. No KF boom or other KF coupling required, implicitly because at 200 tons or below, these vessels are small enough to ride shotgun in their mothership's KF field.
Aerospace fighter bays, 'Mech bays, cargo bays and everything else: See Small Craft Bay.
Why it is impossible to carry spacecraft over 200 tons as regular cargo? Has been an inside joke for KF physicists for centuries. You probably can, but you cannot launch vehicles or spacecraft from your cargo bay. To do that, you'd need a DropShuttle Bay or Small Craft Bay.
You apparently cannot carry DropShips as cargo. Fan theory is that their KF-boom Collars require them to have a similar germanium mesh structure in their fuselage as KF drive cores, so unless properly linked to a hardpoint they'll play havoc with your KF field much like nearby drive core.
(Another fan theory was that perhaps the "magically effective" transit drives of spacecraft have some KF field effect.)