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Yes, Single appliance instance can serve multiple vCenter servers if they are on the same geographical site. If not we do advice to install a separate collector for each geo site.
Skyline is a SAAS offering and Advisor is hosted by VMware CSP. You will be accessing it over the internet.
Thank you so much for clarifications, Ajay.
Your answer looks perfect, however I am exploring & studying the Skyline. If I will need any further clarifications, I will ask in this same thread.