'Cash purchases' are any payments you make to suppliers with whom you do not have an account - so you have to pay for the goods when you buy them. For example, you might buy drawing office items from a local cash and carry.
Cash purchases only include items that you buy for use in providing architectural services. They don't include things like tea and coffee bought for use at work, or items like business stationery. Don't include things like this here. They're covered elsewhere in the cash flow.
Remember to keep safely all the invoices and receipts for cash purchases, even if the amounts are small.