What is the “Label (key)” field for?

For input and choice components, the Label (key) (or “name”) is the key under which the value is sent in the flow response (e.g. screen_0_name_0). It is derived from the label (lowercase, spaces to underscores) but you can edit it. Use a short, unique key (e.g. email, appointment_date). Meta has a limit (e.g. 64 characters) for this key.