Linux Kernel Input Events has the latest input event codes and type definitions.
Linux Multi-Touch Events documents the latest multi-touch event protocol, event codes, and type definitions.
Please note that a document is generated for each kernel release. You only need to change the "latest" in the above links to the version number you are interested in to see the definitions for your specific kernel. For example, replacing latest to v5.4 would show you the definitions for kernel version 5.4.
Wiki Pages at Linux Wacom Open Source Project
The Wacom Kernel Events page describes the kernel input events that Wacom tablets report. It also describes how those events are reported.
To verify kernel events, refer to Wacom Kernel Data.
Refer to Analyzing Kernel Events to view and analyze tablet events.