The Form Control being used in that example is the "Pages" control which defines ad-hoc related Pages for the current Page.
![Pages Form Control definition](https://devnet.kentico.com/cms/getattachment/6f711a4c-fae2-414a-8d14-fee853afbfa0/Screenshot-2022-09-10-120946.jpg)
When you select it as the Form Control to use for a Page Type field (the field must be set to the "Pages" data type) you can configure maximum of related pages you want.
![Pages Form Control in use for a Page Type field](https://devnet.kentico.com/cms/getattachment/3980e893-bd96-44df-805b-bba41f7f4e18/Screenshot-2022-09-10-121315.jpg)