It is a well-known discretionary attribute. It is used to notify downstream routers that a loss of path information has occured.
So the BGP router is advertising an aggregate address to downstream routers and the way it signifies that it has aggregated the address is to set the atomic aggregate.
Atomic aggregate is well known discretionary attribute of BGP.
When a BGP speaking router aggregates or summarises a group of specific routes into a less specific aggregate or summary, it attaches an atomic aggregate attribute to the aggregate or summary.
