[TYPO3-english] What are layout1, layout2, excetera?

[Glenn S]

Relating to Typo3 7.6.

Most records in TYPO 3 appear to have an option 'type', which can take the following values:

default
layout1
layout2
layout3
layout4
layout5
hidden

However, I haven't been able to find anything in the documentation explaining what these are.

Through trial and error I've found that they at least control the various <h> tags for headers.

So, my questions:

1. Is this feature documented somewhere? If so, what is it called (what search term should I use)?
2. Where can I find the definition of their behaviour (what HTML will be spat out for each layout)?
3. I've created an extension using Fluid templates to control the style and layout of my website. Should I pay attention to this layout1, layout2, ... feature, or is it considered an obsolete way of doing things now?

Thanks,
Glenn