The message seems pretty clear, to me at least, but here's my version:
- Project Parameters can be used in schedules but not for tags
- Shared Parameters can be used in schedules AND tags
If the information you care about is only expected to appear in a schedule then a Project parameter will suffice, it will work, it will give you the result you are after. If there is the slightest chance that someone will then ask for the same information to be used in a tag then you need to plan for a Shared parameter. You might consider only working with Shared Parameters for this reason, it's hard to out guess others.
This post "Shared Parameter File - A Little Clarification" revisits parameters and provides a summary of links to other posts I've written over the years. I've also copied them here:
What are Parameters and Why Should I Care?
Sharing Parameters Overview (Part 1)
Walking on Thin Ice
Making a Shared Parameter File (Part 2)
Shared Parameters Part 3
Shared Parameters Part 4
Ignore Good Advice
Home for Unwanted Doors