Are NSF RUI proposals evaluated separately or are they evaluated against the general pool for that program/division/directorate?
At least in mathematics (and presumably in other fields, although I have no direct experience with that), they are evaluated in the same pool as all the other proposals.  In practice, there seem to be two key differences in how they are treated:


Panel members can be a little more forgiving for certain aspects of RUI proposals.  For example, if the PI publishes excellent papers but not very many of them, then this low publication rate would probably be considered a bigger drawback in a non-RUI proposal.
The NSF may give some degree of preference to a few RUI proposals, as described below.


The review panel classifies proposals into three categories: roughly 10% that are highly recommended for funding, 30-40% that are recommended for funding, and 50-60% that are not.  The available funding is never sufficient to cover all the proposals recommended for funding, so further decisions need to be made after the panel.

The ones not recommended for funding are automatically eliminated, and the ones highly recommended for funding are more or less guaranteed to be funded.  The ones in the middle are ranked in order by the panel, but the NSF is not required to follow this ranking.  They largely follow it, but they adjust it based on their own criteria (for example, balance of fields, geographical diversity, etc.).  My understanding is that if no RUI proposal in a given panel would otherwise be funded, then the highest-ranked RUI proposal may be bumped up in the rankings.  However, this is not guaranteed, and it depends on having a high enough ranking from the panel.  The program officers may also compare how panels in different areas ranked their RUI proposals, to see which ones seem most worthy of funding.  However, there is no RUI-only panel.