The ['types'] system is powerful and allows differently shaped editing forms re-using fields, having own fields
for specific forms and arranging fields differently on top of a single database table. The
tt_content with all
its different content elements is a good example on what can be done with ['types'].
The basic ['types'] structure looks like this:
'types' => [ '0' => [ 'showitem' => 'aField, anotherField', ], 'anotherType' => [ 'showitem' => 'aField, aDifferentField', ], ],
So, the basic array has a key field with type names (here '0', and 'anotherType'), with a series of possible properties each, most importantly the showitem property.