Base rewards are the isk rewards per kill for combat campaigns or the isk per plex for plexing campaigns.
The ISK amount field should contain the ISK per kill / plex for each participant or the amount for a split pot.
A split pot is used instead of a per kill/plex amount. For example, setting a split pot with a value of 2,500,000,000 would mean 2.5B would be split between all those involved in your campaign based on their involvement.
IMPORTANT: Standings affect base rewards to a maximum of 10% with the representing agent / agency.