It's possible that you've encountered this term in a specific context, perhaps a fictional work or a specialized jargon.
To help me understand what you're looking for, please provide more context. For example:
* Where did you encounter the term "plainsphere"? (e.g., book, website, conversation)
* What was the context surrounding the term? (e.g., what was being discussed?)
* Did the source provide any further explanation or definition of the term?
With more information, I might be able to understand what "plainsphere" refers to in your specific context.
