An allocation is an actual C8 "order-to-be" provided by GM to a dealer. It means GM thinks it likely it'll be able to produce the car. The allocation followed by an order submitted to GM by your dealer, means you've gone to 3000 status (order accepted for production at the Bowling Green plant in Kentucky). That's my understanding. From there you enter the race towards having your C8 built. As your order progresses through the system, it's assigned a different status code every step of the way as it gets closer to actual production then delivery. Google the GM production codes or check this Forum for info about them. Then ask your dealer what your code status is, from time to time. You can ask elsewhere for that info but dealers usually have access to the best info. Or at least they do if they know the system and they're willing to make the effort and provide the info to you.
If history repeats itself, GM will likely continue production until those at 3000 status have had their C8's built...so in general, once you've got your allocation and order put in, you can expect to get a C8. That's what my dealer told me last year and that's what happened for my 2020 C8. However COVID presents an unknown fly in the ointment, since delays from part shortages have already occurred and likely will in future until the manufacturing process is back on track and not being effected by COVID delays. Plus GM has announced it may not extend the manufacturing time for the 2021 C8's like it did for the 2020 C8's. So it won't make the 2021's into December (as done for the 2020's) and that may affect things. But the expectation now is that anyone at 3000 status, will likely see their C8 built.
You can find this info scattered about these Forums but this gives you a summary of sorts. Hope this helps.