Explain the difference between an aggregate step and a FIXED LOD in Prep.

An aggregate step changes the grain of the data. Group by region, sum revenue, and you get one row per region — the detail rows are gone.

A FIXED LOD calculation keeps every row and attaches the group's value to each one. {FIXED [Customer ID] : SUM([Revenue])} gives every order row its customer's lifetime value.

It is the same distinction as GROUP BY versus a window function in SQL, or agg versus transform in pandas.

Practically: aggregate step when you want a summary output, FIXED when downstream steps still need row-level detail.