It might symbolise the final step of total and utter enslavement, being openly branded like cattle across the entire world, every man, woman and child. Every barcode in the world has '666' already encoded in it, yup that's correct, all barcodes have it. Imagine having a barcode that can be scanned physically branded, tattooed or imprinted on yours and every other citizen's forehead, just like a cow being branded for slaughter. Theres already an RFID chip with 666 in the patent too. So it might be physical branding on the forehead and a chip implanted in the hand for buying and selling, linkes to social credit system and one world digital cashless currency. You've got to remember aliens and fallen angels are pretty much interchangeable at this point. Alien invasion and enslavement is a long and tedious process, there's multiple 'intergalactic' and universal laws that have to be followed, occultic and esoteric one's too. That's why they have to tell us their plan's before they do them. Planets also need to liberate themselves with minimal outside help once they start to get enslaved badly because of the rules. And I'd argue the mark of the beast might possibly be the final step to solidify total enslavement. Once that happens and people accept it, I'd say it means eternal enslavement for our planet and outside positive forces (the good aliens) might struggle to help us, essentially meaning hell on Earth forever. This is all theoretical so I could be wrong. I'd say it's possible to avoid the mark of the beast altogether if we avoid WW3. According to Albert Pike, his prophecy says a third world war is necessary for the Antichrist to come and thus the mark of the beast. I believe it's entirely avoidable, consider the Bible an early warning. In fact, check out the movie from the 70s or 80s called "Early Warning", it's a scene with the mark of the beast being a barcode imprinted on the right hand and warns about the mark of the beast.