Hi Sanjyot,
The line you're talking about doesn't actually do anything on the Masters. It can be safely ignored. We should probably remove it.
To get more technical, that line controls the page height for specific pages. You can have Page 1 be 750px tall, and Page 2 be 1000px tall. The Master, which can be present on any page, doesn't have a fixed height, it uses the RegularPage height that it is attached to.
If you go and view any RegularPage, and return to the Master, you'll see that the line has changed its position.
Here's a video if you're curious -
https://share.smartbuilder.com/public/support/page-height.mp4