The advertisement rules when advertise-external is enabled can be summarized as follows:
If a router has advertise-external enabled and its best overall route is an internal route then this best route should be advertised to:
all iBGP RR clients (if the route came from a non-client peer) or all iBGP non-clients (if the route came from a client peer).
all eBGP peers
If there is a best external route, it should be sent to iBGP client and non-client peers instead of the best overall route.
If a router has advertise-external enabled and its best overall route is an external route then this best route should be advertised to all iBGP peers and all eBGP peers.