The business and above plan let you specify where the checks are coming from and you can pick US, EU, or Asia. Otherwise, it's from the US or another location based on load balancing availability at the moment the check is made.
This feature enables you to check a website as if you were browsing from a specific geographic location, so for example, if you want to check a website that displays prices in USD for US visitors, you can specify that we should visit that web page from the USA.