An Outlier is a human of importance to the Hosts to be experimented as a biological weapon against their enemies. These people are sequestered and hunted by the Intelligence Directorate, and not allowed to be killed by the drones. There are millions of people the Hosts seek out, for they need an army to face a greater enemy.
In special cases outlier were allowed to serve occupation instead of being taken to pods and send to space. One example of this way was when Will Bowman was instead recruited by then Proxy Governor Alan Snyder to Special Police Force unit of Los Angeles bloc. He fastly as former Army Special Force soldier showed superior skills against all resistant groups whose members were mostly just normal people without any military background, which is also reason why Hosts wanted people people like Bowman with elite soldier background to their human army.
Will Bowman and Eric Broussard are among the people the drones weren't allowed to obliterate. Everyone stored in the pods who were sent to space are part of the Hosts secret army. Outliers hold special skills and are trained from various military groups worldwide. Especially Military Special Force soldiers are extremally wanted Outlier type like, such as Will Bowman and Eric Broussard.
After destruction of The Factory, remain Outliers were sent to secret facility in Seattle, which had over 400 000 outliers. It is safe to assume that The Factory had much greater numbers since moving outliers to Seattle started mostly after The Factory was destroyed and by that time most Outliers had been captured and sent to space.