[TYPO3] Problem understanding $TCA['tt_content']['types']

[micho edo]

Hi,
I don´t understand the meaning of the elements of 
$TCA['tt_content']['types']

In some examples, definitions are inserted under the key ['list']
for example $TCA['tt_content']['types']['list'].xxxxxx

What´s defined in the layer direct after $TCA['tt_content']['types'] 
(here ['list'])

Where is this key 'list' used