An aggregate route is a route you define but which is not used for forwarding traffic (next-hop is discard or reject).
Only if there are more specific routes (contributing routes) will the aggregate be active in the routing table and therefore available for use in the export policy.
root@SRX# set routing-options aggregate route 2.2.0.0/16
root@SRX> show route protocol aggregate detail
inet.0: 6 destinations, 6 routes (6 active, 0 holddown, 0 hidden)
2.2.0.0/16 (1 entry, 1 announced)
*Aggregate Preference: 130
Next hop type: Reject
Address: 0x11717f0
Next-hop reference count: 3
State: