Great question. Should definitely be documented.
Also I wish it could summarize several fields for one layer. I tried doing summary, average, min and max for Sales Price in a Parcel polygon layer. It has a bug, where it can not do more than one field for a layer. It simply sets them all to the first one, whether it was sum, average, or minimum, etc. Same for the Label. field, it repeats the first text string for all of them upon clicking OK. At least, that is what happened for me and Tech Support in repro. I got a tracking # BUG-000116103. Hopefully resolved in WAB 3.0. And remove Incident Widget if that is the older version.