Run Across All Exchanges/Run-Of-Network (RON) refers to when a campaign is applied to a wide collection of websites/massive volume of impressions. It is important to ensure specific targeting options are applied to a campaign before using RAE/RON (i.e. contextual/audience segments, hyperlocal targeting, retargeting/audience list) to prevent your spend cap/budget from being quickly exceeded.
Once Run Across All Exchanges/RON has been enabled, and the campaign has been launched, you cannot change or revert back to individual site selection. To do this you would have to create a new campaign.