This item lacks wiki-assigned association. It is permanently available. Added in unknown version.
Special Outfit Coupon is an item in Honkai Impact 3rd.
It can be used to purchase 4-star outfits (with the exception of Honkai World Diva, which is 5-star) in Outfits > Special. Those outfits are, normally, not obtainable for free via events.
Sources
B-Chip Bundles
Legacy
With the release of Version 3.0 which introduced B-Chips, all previously owned ADV Outfit Coupons expired and were converted 1:1 into Special Outfit Coupons.
Time-limited
Outfit Supply - often appears as one of the possible drops.
Sell Price: None
Honkai World Diva, which is 5-star) in Outfits > Special. Those outfits are, normally, not obtainable for free via events.
B-Chips, all previously owned 
ADV Outfit Coupons expired and were converted 1:1 into 
Special Outfit Coupons.
Silky Year Outfit Option.