A question about reverse polarity protection diode
Yes, this diode is probably for reverse polarity protection. It does its job but has the disadvantage that about 0.6 V is dropped across it. It needs to be rated for the current, both continuous and surge, as well as the reverse voltage it might encounter. 1N4148 is fine for lower current applications. A shunt diode with polyswitch/fuse is also an option. More elaborate schemes with MOSFETs are possible.