How to interpret "adjective and adjective" structure?

Solution 1:

The question relates to simple set theory. Two sets of projects are relevant: key, and large.

This means that there are three classes of project: key, large, key AND large. Only if all key are large and all large are key does the number of classes reduce to one. If all large are key or all key are large, the number of classes reduces to two. Otherwise, the number of classes is three, regardless of any partial relationship between key and large projects.

This means that, without any context, further detail, or rephrasing of the sentence to clarify the 1 or 2 class possibilities, the only prudent interpretation is that they are responsible for all or any of the three classes.