Hi Colin500,
The standard VRF import and export policies affect remote PE prefix leaking. Meaning, import policy on a local PE tries to match the route-target applied via export policy on remote PE and if there is a match it installs that route in vpn.inet.0 table.
However, scope of auto-export is local. It doesn't affect the remote PE prefix learning. When you configure auto-export, the vrf-import and vrf-export policies are compared across all VRF instances on LOCAL PE. If there is a common route target community between the instances, the routes are shared.
Here is an excellent blog on auto-export:
https://forums.juniper.net/t5/Archive/Using-rib-groups-or-auto-export-for-route-leaking/ba-p/202349
Here is an example on practical use of auto-export:
http://switchpacket.blogspot.com/2014/03/juniper-route-leaking-part-3-vrf-route.html
PS: Please accept my response as solution if it answers your query, kudos are appreciated too!
Thanks
Vishal