Hi Nancy, I see that you're a LEEDuser member, which is great since you are new to LEED. There is a lot of information on the details of each NC-v2.2 LEED certification credit here that I would recommend you read up on—including MRc4. We also have a specific forum for MRc4. There is more detail on the MRc4 page, but the difference is whether you are using for your materials budget the actual project materials cost (most projects do this), or a default value of 45% of the project cost (this can be particularly helpful for renovation projects).