A breakdown is a dimension you slice a KPI by: the people, places, products, services or projects you record data against. Breakdowns turn one flat number into something you can compare, so "Revenue" becomes revenue per region, per product, or per salesperson.
Breakdowns and items
Every breakdown holds items. The breakdown is the category; the items are the entries inside it. A Projects breakdown might hold the items Speaker Project and Nike Project. A Regions breakdown might hold North, South and East. You build the list once, then reuse it across as many KPIs as you like.
Why they matter
Breakdowns are what let you see where a number comes from, not just the total. Once a KPI is split by a breakdown, the KPI Sheet gives you a row to enter a value for each item, and your dashboards and reports can chart the KPI one item at a time or compare items side by side.
You can split a single KPI by up to four breakdowns at once (say Region then Product) and then choose the exact combinations you actually track. You do that on the KPI itself, in the Breakdowns & sets panel, using the breakdowns you've created here.
The People breakdown
One breakdown, People, is built in. It mirrors your login users automatically so you can track a KPI per person without retyping names. You can rename it, but you can't delete it, and a person linked to an active login user can't be removed while that user exists.
Keep breakdowns broad and reusable: Products rather than Q1 Products. A good breakdown is a dimension you'll want to slice several KPIs by, not a one-off list. Deleting a breakdown only hides it from pickers; your historical entries are always preserved.