Custom Rules are the future. They are way more flexible, based on the new ruleset engine, can skip more items (Allow is now “skip”, and lets you skip specific features). There’s a migration guide here: Firewall rules are becoming custom rules · Cloudflare Web Application Firewall (WAF) docs
It looks like due to your special circumstances/glitch they enabled both to display for that one zone. Normally though, they are slowly automatically migrating all Firewall Rules into Custom Rules, and hiding the Firewall Rules Tab. Eventually, your other zones/domains will be migrated as well.