How is the virtual address space up to 64 TB calculated?

I searched a lot of posts, but this article makes sense:

Http://bbs.chinaunix.net/forum.php? Mod = viewthread & tid = 1959427 & page = 5

The virtual address space consists of the global address space mapped by GDT and the local address space mapped by LDT.

The index part of the selector is represented by 13 BITs, plus one bits that distinguish GDT from LDT;

Therefore, Intel 80X86 CPUs can index A total of 16384 selector characters. If the maximum length of each segment is 4 GB, the maximum virtual address space range is 16384*4G = 64 TB.

I just don't know what this 64 TB can do.