Residential (commonly referred to as 'resis') are for generation, scraping, and other tasks. Residential proxies go by data, meaning you purchase gigabytes of data at a time - this data gradually runs out upon usage and must be topped up. ISP proxies are for checkout, spoofing, queueing, and other tasks - these proxies remain the same for the entire month (or day if you purchased a daily plan). It's important to have a mix of both to maximize your success.