How to implement PTC RESET FUSE

Hello A newbie is here:) I am currently designing a board similar to Arduino. I have a PTC fuse that I would like to use on my voltage line of the USB port, but I realized there is also a capacitor near it which decouples the fuse to ground. Is it necessary? Any special value ? Because I saw Arduino nano has 4.7uF whereas Diecimila has 100nF.

Furthermore, Regarding the diode that we use to prevent currents in wrong directions. Should that diode be placed near the USB port itself? does that even matter?

Thanks in Advance

The cap decouples the power supply line to Gnd.
The PTC fuse attempts to keep your PC supply safe.
The diode by the barrel jack connector? That prevents a power supply from being connected backwards and blowing up the whole board (Diecemala).
The diode on the Nano prevents an external 5V source from driving into your PC, which PCs don't like, and may report as a surge and shut the USB port down (on my Lenovo that's what happens anyway) until the PC is reset.

Thank you sir, your helps in this from are invaluable. May I ask does it matter where to place the decoupling caps on the PCB? should they be near where power lines start? or near the ICs perhaps? Also, as for the diode, I have seen there are two decoupling caps each side of it, that one is necessary I assume?

All decoupling capacitors should be placed as close to the component as possible and the wires to it should be as short as possible.

A capacitor either side of the diode helps make it a better circuit. Diodes generate noise and the cap helps to suppress it.

Thank you Mike. Now it make sense