壹 is the formal/financial 'capital' form of
一 (yī — one), used on cheques, contracts and official documents to prevent forgery. The full set:
零壹贰叁肆伍陆柒捌玖 (líng yī èr sān sì wǔ liù qī bā jiǔ).
零壹 thus is just '01' rendered in tamper-resistant formal script.
