System under test

On a higher level you can have a variety that is based on something like this:

System to be tested ▼
- Web Interface ▼
  - With a third party ▼
    - With hardware ▼
      - Test Option A
    - Without hardware ▼
      - Test Option B
  - Without a third party ▼
    - With hardware ▼
      - Test Option C
    - Without hardware ▼
      - Test Option D
- Operating System Interface ▼
  - With a third party ▼
    - With hardware ▼
      - Test Option E
    - Without hardware ▼
      - Test Option F
  - Without a third party ▼
    - With hardware ▼
      - Test Option G
    - Without hardware ▼
      - Test Option H

However, on a lower level, you probably test something like a module or a new functionality...

On a even lower level, you probably test something like a bug of that module or new functionality...

Once you have decided what is going to be tested, where the focus will be on, then you can determine what amount of attention it will get

And next how you are going to test it.

And this whole System(s), big Module, average Functionality or small bug can be systematically checked in the Definition of Done.