Declared tariff is the published rack rate of the room — what you would charge if the customer paid full price. Indian GST law (CBIC Circular 47/21/2018-GST) uses declared tariff, not the discounted transaction price, to set the GST slab.
For hotels with composite supplies (room + meal plan), declared tariff includes the room rate plus the meal plan rate multiplied by the number of adults. Using room-rate alone underdeclares and creates audit exposure.