They've announced how the "Premium Currency" works mentioned on the store page stuff that was revealed alongside this trailer:
"Store FAQ
What's in the premium store?
The premium store is focused on cosmetic items, that is, character skins, weapon skins, and other smaller customizable items, as well as DLC content.
How does premium currency work?
Aquilas are the currency used in the Premium Store to purchase any of the items available in it. Aquilas are currently only obtainable via in-game purchase.
How does in-game currency (Tertium Twins) work?
Twins are a currency earned by doing activities in game, such as completing missions or selling salvaged items. This currency is used for other ingame activities such as improving equipment. This currency is not usable for acquiring cosmetics from the premium store.
How can I get cosmetics?
There 2 ways of getting cosmetics: progressing through the game or acquiring them through the premium store. Progress cosmetics will be given out as the player progresses through the story and the different challenges the game offers. Premium store cosmetics can be acquired as long as they're available and can be equipped by any character of that class, account-wide.
How can I manage my cosmetics?
Cosmetics are parts of sets, which means they usually have matching parts that make for some really cool looks, however, players are free to swap around each part independently and make their own looks."
What is Darktide? 4 player Co-Op Action Adventure in the Warhammer 40K Universe. If you've played Vermintide, think Vermintide in 40K flavor. When is Darktide coming out? November 30th 2022. I don’t know much about 40K - what should I check out or read up on? Some great reads to get you familiar...
steamcommunity.com
I know microtransactions are common in games now and vermintide 2 had similar, but I still really dislike the idea of locking off cosmetics behind a paywall arbitrarily, especially in a game said to be so focused on customization and creating you character. It's still content where they've made a decision to negatively affect players at no benefit to them.