From most users' perspective, they're the same thing! For Watchmark to manage a physical door, you install a Watchmark access panel nearby and wire it up to an electric strike or maglock that either holds the door shut or allows it to open.
So there's exactly one access panel per door, and the access panel connects to both the internet and power over an ethernet cable. Once it has internet and power, the access panel will connect to the cloud over an encrypted connection and download all the schedules/users/passes/services/etc that it needs to function, and when an authenticated user needs access, the access panel will unlock the door.
So the only difference is that, in the documentation, we'll refer to an "access panel" when we want to make it very clear we're talking about the piece of Watchmark hardware you install in the wall rather than the logical "Door" we usually talk about which is really a physical door-plus-access-panel combo.