This directory primarily contains php-scripts which are meant as 'external modules' as opposed to features included in the typo3/sysext/cms/tslib/ libraries. Although they are distributed with TYPO3 just like tslib/ they form a basis for externally developed front-end functionality. So for most of these scripts, be inspired by them to write your own code. Notice the word 'most'; because some are written long time ago and does not represent the state-of-the-day to do it.