Need help understanding units and efficiency values
 

I'm trying to understand the proper way to account for the fact that most of the assigned people to a project can only spend about 80% of their working hours working on the project. I've been looking at the units and efficiency values for the people resources but I'm not sure how those values are used by OP. Note, I can not find one instance of the word "efficiency" in the OP manual.
Edit: actually I was able to find one instance but it wasn't helpful.

Related to this, I've tried setting the efficiency value to 80% for all the people working on tasks but when I select a particular task assigned to one person the task inspector shows the Assigned Amount as 100% of 100%. What is this telling me?