壹 is the financial/formal uppercase form of the number one (一), used on checks, receipts, invoices, contracts, and any document where tampering would be costly. It's part of a standard set of complex numerals (贰, 叁, 肆, etc.) that prevent forgery because the strokes are harder to modify than the simple characters. You'll almost never see 壹 in everyday writing — only when the amount matters legally.