What does it mean that classification is a compression technique?

Categories reduce complexity by treating similar things as equivalent for a given purpose.

Example: A hospital emergency department triages patients into five categories: resuscitation, emergency, urgent, semi-urgent, non-urgent. Each patient is unique — different symptoms, histories, pain tolerances, comorbidities. But the triage system compresses all of that into five buckets that determine who gets seen first. That compression loses detail (the patient's full story) but gains something critical: the ability to allocate scarce resources without drowning in case-by-case analysis of 200 people in the waiting room.

Try this: Pick one classification system you use daily — your email labels, your task priorities, your contact groups. Write down three things that system compresses away (details it ignores) and three things it preserves (distinctions it keeps). Then ask: is the compression ratio right? Are you losing information that actually matters for your decisions, or are you carrying detail you never use?