From Halopedia, the Halo wiki
The Exchange is a rotating storefront featured in Halo: The Master Chief Collection, in which cosmetic items can be purchased with excess Season Points earned in-game. The Exchange is used as a means to reintroduce items previously only unlockable via time-limited challenges, but will also feature entirely new items that did not appear in previous seasons or their challenges. The Exchange has two categories, limited-time items which are rotated each week, and featured items, which remain available for longer. Each week, the following week's items can be previewed.
The Exchange was first tested during the Flights for Season 6: Raven, during which time the Exchange contained similar content to its final release with a notable inclusion; the Avalanche skin originally unlockable in Season 4: Reclaimer as part of a ranked challenge. However, community pushback on this led to the skin being removed for final release, with 343 Industries opting to leave more time and choose a "more appropriate" price point for the ranked reward skins before including them in the Exchange system.
The following table lists each week's Exchange items, starting from when it was introduced on April 7, 2021.
Season 6: Raven
Season 7: Elite
Season 8: Mythic