What is the difference between airdrop and retro drop?

Could you please explain the distinction between an airdrop and a retro drop in the world of cryptocurrency? I'm curious to know how these two distribution methods vary in terms of their mechanics, purpose, and potential benefits or drawbacks for both the project and the recipients. Specifically, how do they differ in terms of the way tokens or coins are disseminated, and what are the main motivations behind using each method?