How it works
Summarize takes fields and assigns each an action: Group By, or an aggregation such as Sum, Count, Count Distinct, Average, Min, Max, First, Last, or Concatenate.
Group By : Region
Sum : Revenue
Count : OrderID
CountDistinct : CustomerIDThat is SELECT region, SUM(revenue), COUNT(order_id), COUNT(DISTINCT customer_id) ... GROUP BY region.
Fields not listed disappear
Any field you do not either group by or aggregate is dropped from the output. Alteryx does not warn — it simply is not there downstream.
If you need a descriptive field to survive, either group by it too (safe when it is functionally dependent on the group key) or use First as its aggregation.
Concatenate
The Concatenate action joins text values within a group into one string with a separator. Useful for rolling up a list of products per order into a single cell — awkward in SQL, one setting here.
Summarize before Join
The standard use beyond reporting is preventing a fan-out. Summarize the many-side by the join key so it has one row per key, then join. Same pattern as a pre-aggregated subquery in SQL.
Multi-Row Formula
Where you need a running total or a comparison with the previous row, that is Multi-Row Formula, not Summarize. It can reference [Row-1:FieldName] and can group by a field, which makes it the equivalent of a SQL window function.
It depends on row order, so put a Sort before it.