A Customisation Developer primarily modifies or customises software applications according to specific client needs. They work closely with software designers, business analysts, and end-users to understand requirements, then use programming languages and tools to tailor the application. Their responsibilities also include testing the customised software for bugs, ensuring compatibility with existing systems, and updating the software as necessary. They must also document changes made to the software for future reference. Their goal is to enhance user experience and functionality, while maintaining the integrity and performance of the original software.