I discovered a new vertical metric, what is it?

I looked into a font to see how it was build because I was intrigued by its huge leading
it looks like it uses a large blank space below the ascender to force that huge leading.
The whole block is 1000 units from top to bottom. So I am really curious to know what is this extra bottom metrics!?

The “descender” is a zone, the line on the bottom is the actual descender.