Once prefill is enabled, you can decide to prefill from records only if certain conditions are met.
Prefill is applied at the object level - not on a per field basis. For example, you can set conditions to prefill an object (or to not prefill it) but you cannot set a condition to prefill a specific field. You can use javascript to clear a field that has been prefilled to allow a new value to be provided by the user.
The prefill conditions are executed against data in Salesforce at the time the prefilled form is loaded. The prefill conditions will not take into account the state of any data on the form or set using the Javascript API.
All fields on form objects can be included in prefill conditions even if the fields are not located on the form.
Conditional Prefill can be configured on all forms (including NativeCloud and Community Forms).
In Community Forms, you will notice options to prefill by Logged-in User and also by Record List.
💡 If an object is not prefilled due to a condition, then all of its children and Lookups (parents) will not be prefilled either. In this case, objects not prefilled will create new records and the relationship to prefilled records will reference or change to the newly created records.
Each object that ends up being prefilled will consume an API call. If there are multiple records to prefill a Repeated Child object on form, that will still consume a single API call into Salesforce.
General field data and TextArea field data cannot be used in conditions for prefill.