Thanks for posting your question!
.region files define the layout of your site. If you need more flexibility, you can create multiple .region files so that you can have various layouts for your site. After creating these files and referencing them in the template.conf file, you can change a layout directly on the Squarespace editor, in the Advanced panel of a page. You can read more about layout and regions on our official guide here.
If you have additional questions, feel free to leave a comment!