Custom Pages have quite a few capabilities. Namely, you can use the basic What You See Is What You Get Page Editor, the Page Designer, the Inline Editor and the Code Editor. So when selecting the Custom Page type, you have the following capabilities:
1. Use the simple Page Editor, that we also call the WYSIWYG editor (what you see is what you get) to create a simple, basic page that uses the design settings set within the design tab of your subdomain settings
2. From the Page Editor, you can access the Page Designer by clicking on the Page Designer button present at the top of the page. This will take you into Page Designer where you can select and add content blocks to create the layout and structure of the page. You can edit and update the text, images and content within the design blocks.
3. The Inline Editor then allows you to either edit/update the content of the design blocks you have created with the Page Designer OR allows you to edit a HTML template that you have uploaded accordingly. You must upload the source files of the template into your source file hosting and then you can add each HTML index page that your template contains into the custom pages BODY section or by simply dragging and dropping the HTML index page into the custom pages drop as seen here: