The stock id of the current website from the cache. Core resolves it with two selects on every request that loads a product collection.
Plugin/StockIdFromCache answers
Magento\InventoryCatalog\Model\GetStockIdForCurrentWebsite::execute() from the entry
FASTBOOT_STOCK_ID_<website code>, under the stock_id switch.
Plugin/ForgetStockIds cleans the entries after a write of a
stock's sales channel links.