the fs 5u internally is a momentary switch. Meaning that the switch in it, when you step on it,, a connection will be made, when you release the stepping, the connection will be cut off.
The "U" means that the foot switch is an unlatched/momentary type, step on it to "on" and to "off" it, just release the stepping from the switch"
Wheras the "L" in FS 5L means that the switch is a latched type, just like the stompswitch in our effect pedal, step one time to "on", step another to "off". Similar to the lighting switch found in houses and various electronic devices/toys etc
and for the connection of the switch internally, theres 2 kind usually, a normally open status or normally close status. Usually mentioned as "NO" or "NC" in engineering context.
for normally open, it just mean that when you dont step or dont press the switch, there is no connection to what ever the switch is connected to. When you step on it, it will establish the connection. So its like an on/off switch
for normally close, it means that the switch is always "on", until you step on it, then the connection will be cut/'off"
the polarity switch is for the 2 things above, to change the switch from normally open status to nomally close or vice versa. So in either stage, changing the polarity is affecting the switching status when you step on the thing and when you dont step on it
as for what it is used for, that will have to depend on the device that you are connecting the footswitch to. Whether it need a normally open or normally close footswitch to run certain functions within the device. Eg are tap tempo, on/off record status in hardware recorder, sustain pedal for keyboards etc.