Darktide Penances guide

Darktide Penances can be a bit of a mystery at first, as you'll be receiving points from seemingly out of nowhere when you're bashing heads in Tertium. You'll gain a lot of these points if your missions from the Emperor go well, but what are they, and can they be spent on anything?
If you've been successful in clearing out Pox Walkers from the Tertium underground, then you'll no doubt have a small fortune of Penance points at your disposal. There are multiple forms of currency in the co-op game, with gold perhaps being the most useful of all as it enables you to purchase some of the best Darktide weapons in the armoury, but what can you spend your Penance points on?
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