Changing a Default Region or Location means that every new access method, both AWS or AZURE, will present the selected Region (AWS) or Location (AZURE) as the one defined by default in the select option of the form.
Also, a default Region/Location will be used to patch old sessions prior to release 0.4.3, to have it as the default one.
Every Service Provider call done with the active session will be directed (if possible) in the selected region.
Where to change default Region/Location
Both for active and inactive sessions you can access the change region modal by clicking on a session's menu button:
And by selecting "Change Region/Location" (depending on the type of Session). Here you'll get into a modal to change the current region for a session.
Note: if the session is active, the actual credentials will be rotated by new ones that reflect the newly selected region.