The backend forms are defined in the 'types' section. Here we see the familiar items such as 'header', 'text', 'textpic' (text with images). The 'showitem' definition contains the parts which are displayed.
In really old versions of TYPO3 CMS there were no tabs in the backend forms. There were sections which were indicated by the
--div-- separator in the TCA. Later on an option was introduced ('dividers2tabs') to transform the dividers into a tabbed dialogue.
Palettes were a feature to have extra (less important) fields in a group which could be made visible with the click on an icon. Power users often like to have all options available and use the checkbox 'Show secondary option (palettes)' to have all fields visible all the time.
Although the tabbed dialogue made the page less cluttered it was often needed to jump from tab to tab in order to find the option you wanted.
With TYPO3 4.5 the backend forms were reconfigured. Items that belonged together were grouped and the styling got more uniform.
Divs are now turned into tabs by default and the palettes in the core are forced to be expanded all the time. Palettes of the standard fields in content elements are now used only to group fields because they cannot be collapsed.
With this knowledge we can build custom content elements based on existing or new fields. In a lot of cases there is no need to only new fields to the already large set of fields in tt_content. In the end we just need a solution that's easy for the editor to work with and which isn't a burden to the system.