Zoho Billing was previously known as Zoho Subscriptions. The product itself didn't change in a fundamental way, it still handles recurring billing, invoicing, and subscription management, but the rename reflects a broader scope than the original name suggested.
"Subscriptions" described one part of what the product does well, recurring billing for subscription-based businesses, but the product has grown to also cover one-time billing, quotes, and broader invoicing needs that aren't strictly subscription-based. Zoho Billing is the more accurate name for what the product covers today.