Yes that’s one of the parts we haven’t explained very well yet. It isn’t for powering the AutoPi from a 5V supply, that isn’t possible because the OBD chip requires a 12V supply to detect vehicle health. But you are right that its related to the “Force 5V” marking on the PCB.
The use of the minijumper is probably more relevant to users of the DiY edition or users who want to do some “hardcore” changes on the AutoPi. With the minijumper you can force the AutoPi to maintain the 5V supply on all the time. This means effectively that the Raspberry Pi wont be able to go to sleep, because you disable the Smart Power Manager (SPM) chip on the AutoPi.
The jumper will come in handy when you debug or develop new core features, where you want to change the functionality of the SPM. For normal use it shouldn’t be necessary.
Hope it all made sense.