What is the value shown for memory graph in XenCenter?

I would like to ask question to people in this community about Xen Center.

I use Xen 6.5 in production environment. However, I quite don't understand the value shown in memory graph of "Performance" tab in each server. My purpose is to exactly understand the value used and use for troubleshooting.

Could someone educate me which command does Xen use to calculate the memory, or perhaps script?

Thanks, Yu