relationship

relationship
Type:string
Scope:Display / Proc.
RenderType:all
Default:manyToMany

All possible values are:

oneToOne
Stores the uid of the selected category. When using this relationship, maxitems=1 will automatically be added to the column configuration. While one record can only have a relation to one category, each category can still have a relationship to more then one record.
oneToMany
Stores the uids of selected categories in a comma-separated list.

manyToMany (default):

Uses the intermediate table sys_category_record_mm and only stores the categories count on the local side. This is the use case, which was previously accomplished using ExtensionManagementUtility->makeCategorizable().

In the following example a category tree is displayed, but only one category can be selected. It is

$GLOBALS['TCA'][$myTable]['columns']['mainCategory'] = [
   'config' => [
      'type' => 'category',
      'relationship' => 'oneToOne'
   ]
];

All other relevant options, for example maxitems=1, are being set automatically.