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I am designing my first PCB with an MCU and I see this section in the datasheet that brings up a loading capacitor used for pin parameter measurement.
I don't believe this is load capacitance for a resonator because that value you get from the resonator datasheet and the figure doesn't match the needed connections. From looking around it seems this is related to S parameters for high-speed PCB simulation and verification, but it's hard to tell for me. This is from STM32F446xC datasheet.
This is the capacitance the pin is loaded with before making switching speed measurements. It is intended to represent the sort of loading you would get with a length of PCB track and a few gate inputs, so that you are able to get the specified switching times when using a practical board and fanout. If the manufacturer measured the pin unloaded, then you could never achieve the data sheet switching times with any practical board.
It's not a recommendation of how much capacitance should be on the pin for it to function. It's not even an upper limit of how much can be on the pin, though more capacitance than 50 pF will mean that the pin may not meet the switching times mentioned in the data sheet.
