When a peripheral is to use a (non-analog) pin, this pin has to be configured as Alternate Function in a bitfield in GPIOx_MODER, respective to given pin. This connects the pin to the AF matrix. Then, the appropriate peripheral is to be chosen in this matrix in the given pin's GPIOx_AFR bitfield.
Normally, it is anticipated, that exactly one pin is connected to a given internal pad of the peripheral. However, in the AF matrix, often several alternative pins can be chosen for one peripheral pad. What happens if several pins are connected to one pad simultaneously? And what happens, if *no* pin is connected?
While these settings may be unusual, they may still result in useful configurations. Also, the RM does not appear to exclude such usage explicitly. Unfortunately, the behaviour in these cases is not documented.
When multiple pins are assigned to a single peripheral pad, experimental evidence indicates, that if this pad is input, the individual pin's states are simply ORed. Similar experiment indicates, that state of output pads (both enable and state) are simply fan-out to all assigned pins.
When no pin is assigned to a peripheral pad, outputs are simply ignored, and inputs - according to experimental evidence - are set to 0.