Resource Units by Work Plan or by Task

If at the resource level max units are set to 100%, is there a way to change this on a project by project basis or even by a task basis?  After changes to resource units are made at the project or task level, saved, and published, when you go back into that work plan all the changes revert back to 100%.  I need a better understanding of this functionality and what I may be doing incorrectly.