What are the different types of work in a development team?
Nice question!

Possibly, Changes are further strategic product development beyond current work which is more than required by technical debt? For example, introducing a new framework, or nice abstraction layer. (which could break a lot though)

Internal projects could be custom helper tools to automate something invisible in the backlog. Sometimes you have also to fill in time sheets.

By the way Unplanned work could be also a request to support sales or HR or some unexpected meeting or all hands social skills training event.

There is anyway a sequel to "Phoenix Project" called "Unicorn Project" by same author, focusing on the Dev part of the equation.

I haven't read it myself yet but hope you might find the answer there.