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Thus summarizing and coming back to our point, TYPO3 Customisation would essentially denote - change in the configuration language or altering Typoscripts, changes to the existing Extensions for suiting specific needs, developing new extensions for meeting very complicated and specific requirements, changes to template and integration and further but not the last one, making changes in the Backend Module of the TYPO3 for the need to changing the way that you want the site to look.
TYPO3 is flexible to be customized to any extent and can help you to build simple websites to complex web applications with much lesser time as compared to custom or bespoke software development.