Puffin is entirely correct. Low/Medium is there for future proofing, since they will be differentiated eventually. And "primitive" refers to the state of the code. For example, exos used to have ships all travelling at their default speeds, so they would show up piecemeal and be easily destroyed. This is no longer the case.
I have updated the description for clarity.
The error you saw is from the visual code; it tried to generate a Notification (the popup notifications like wave warnings, etc), and it couldn't find the right graphic for it. That's Chris' department I'm afraid. If you can find a consistent reproduction scenario it would be appreciated!