What is an M/E? Why Would I Want 2? (or 3 or 4?)

What does the term M/E mean when talking about video switchers? Is it something I need? Why would I want to have more than one?

►ATEM 2 M/E Production Studio 4K - https://djp.li/atem2me
►ATEM Constellation Switcher - https://djp.li/constellation

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