GA4 breakdown report: sessions / users / key events (conversions) / total events by a dimension. dimension: channel (default) | source | medium | source_medium | page | landing_page | country | device | date | event. The channel/source breakdown answers 'which traffic converts'.

dimension='event' splits by EVENT NAME, which is the only way to isolate ONE key event. Without it conversions is the SUM of every key event in the property, so a property with purchase + form_sent + mql reports a single number that cannot be reconciled against what an ad platform counted — the most common check in any paid media audit. Use ga4_list_conversions to see which event names are key events.

dimension2 crosses a SECOND dimension with the first, e.g. dimension='event' + dimension2='source_medium' gives form submissions per traffic source. Rows then carry both dimension fields. events is the raw event count and conversions counts only key events, so on an event row the two differ whenever that event is not a key event.