A whitelist is a subjective measure that a blockchain node can put into effect to allow transactions to be put into a block only if they are listed on their author (account) or contract whitelist. Each node can specify their own whitelist, and these are not publicly viewable. Even if a node does not have the author/contract on their whitelist, it will still accept the transaction as valid if another Block Producer places the transaction into a block. To completely block transactions that are not on the whitelist from being added, all Block Producers will need to have the same whitelist settings at all times. Whitelist takes precedence over blacklist. Watch our video explanation.