Supplies are recorded as assets when purchased.Therefore the decrease to the supplies account in the adjustment is for the amount of supplies:

Question

Quizplus · Accepted Answer

The decrease to the supplies account in the adjustment is for the amount of supplies that have been used up during the accounting period. This adjustment is necessary to ensure that the balance of the supplies account accurately reflects the amount of supplies that are still on hand, and therefore available for future use. Any supplies that have been purchased but not yet used would still be recorded as assets on the balance sheet, and would not be included in the adjustment.

Supplies Are Recorded as Assets When Purchased