[TYPO3-mvc] How to extend sys_category correctly?

[Jan Kornblum]

Dear newsgroup,

how do i extend Category (sys_category) correctly? I tried the same 
steps as when extending FrontendUser (fe_users), but anything seems to 
differ (I've used Extension Builder each time):

1. There is no "record type" field shown in the backend for the 
Category records (although this column exists in db)
2. My repository seems to load all categories independent from having a 
right "record type" being set at all and all categories are typeof my 
extended category class (var_dumped)

The configurations for both (extended fe_users and extended 
sys_categories) cases look equal:

1. Mapping in ext_typoscript_setup as done the same way
2. Model is extended the same way
3. TCA is set up the same way
4. Anything forgotten?

The only thing which i've manually adjusted is the parent-property with 
its getters and setters in my extended Category-class 
(\XX\Ext\Domain\Model\Category instead of 
\TYPO3\CMS\Extbase\Domain\Model\Category).

Do i misunderstand anything?

Kind regards, Jan