I'm dumb because I should know this

What's the difference in effect between fixed calipers and floating calipers?
I'm preparing to do a brake job in the front soon, and was reviewing the FSM and noticed that there are fixed calipers in front and floating in rear..

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RIP: 80 300SD
RIP: 79 450SEL
2002 E430 4matic (212,000km)
2002 ML500 'sport'

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