What happens to fields you do not include in a Summarize?

They are dropped from the output, with no warning. Only fields you group by or aggregate survive.

If a descriptive field needs to carry through, either group by it as well — safe when it is functionally dependent on the group key — or give it the First aggregation.

It catches people out because the workflow keeps running and the field simply is not there downstream, which surfaces as a confusing error several tools later.