CareerCup

For this type of testing questions, I think you generally need some assumptions/clarifications prior to answer

e.g.)
What's the usage of the digital pen? - Could be drawing something or writing something etc...
Who are the users? - Could be broad consumers, companies etc...
What's the device? - Could be tablets, mobiles, PCs etc...
What could be the application? - Could be painting software, notepad or etc...
What's the not intended use case?
or something

Then classify the tests from software test perspective

e.g.)

Functional Tests

Normal case:
- You can write/draw something to the screen
- Your drawing/writing can be erased.
- Your drawing/writing should have multiple colors/fonts
- If multiple users try to write/draw at the same time, it should work.
- You can't draw/paint when the app is not running
etc...

Negative case:
- If you light touch on the screen, it's still recognized or not.
- If you touch the screen too hard, what happens??
- If you intend to write/draw something with opposite edge of the pen, what happens??.
etc...

Non-Functional
Compliance:
- The pen can be usable for multiple devices or not e.g.) Tablets, PC, mobiles etc...
- The pen can handle different OS/version of the software
etc...

Performance
- Keep writing/drawing several hours and see if it's still working...
- If it's on online share, try to add maximum users and keep writing/drawing...
etc..

Maybe think about Stress, Security or more...