[exim] Assistance configuring router

I have inherited an Exim 4.x mail relay server that relays our
organization's internal mail from application servers. We now have a
need to write a router in Exim's config that behaves like this:
IF incoming mail is from a specific IP or subnet AND mail has
'[secure]' in the subject line THEN mail needs to be routed to another
specific mail exchanger.

What sections do I need to familiarize myself with in the Exim Specification?

Thank you very much,
Joseph Kezar