Traditional "project scoring" systems we see are a list of projects in a spreadsheet scored against some sort of measurement criteria.
Most project ranking schemes stop at the ranked list of projects, at best. The problem is that you never know if the prioritized list can be completed without knowing what resources it takes to get all the project done, satisfactorily.
These "constraints" must be applied to the project portfolio to determine "what can actually be done." A ranked list of projects is just the first step. This tells you what could be done, but not what can be done given available (and now "diminishing" in some organizations) resources. In addition to cost, material/equipment, and people constraints there are RISKS that need to be considered when selecting the optimal portfolio mix. Further, there are also the MUST HAVES that tend to override the basic logic of constraints. Without factoring in the constraints--just prioritizing projects in a portfolio is only 50% of the solution.
In the example above you can see a simple illustration of a budget constraint being applied to my little project portfolio. I need $10,150,000, but I only have $9.5M. The model applied the constraints and dropped the lowest ranking project. Simple to illustrate the point, but this gets more interesting when more projects are added into the mix and other factors such as RISK and MUST HAVES are factored into the model.