"%CUSTOM( )" is a Dialog Processing Statement. With which you can add custom dialog statements.
It has 2 parts separated by a ":". The first part is the name and the second is a string the statement is replaced with. When the added statement is called it has to end with parenthesis, inside of which you can put a string which will replace all instances of "%VALUE" inside the custom statement string.
Example:
Initialization:
%CUSTOM(PRINT:%LUA(game.print("%VALUE")))
Execution:
%PRINT(Hello World)