Generally, this term is used to describe 'one-off' / 'one-of-a-kind' knives, or a limited run of knives that has been specially made to meet the specifications of a retailer or customer (Typically using material or blade types that are not usually available).
Unlike Factory-Made knives, Custom-Made knives are always available with limited quantity, production is limited and the entire making process is manually operated by more of the custom hand-making process.
Sometimes, these custom knives will be sold exclusively to a specific customer and are not available to anybody else.