In a typical form flow, you only collect answers from questions that appear within the flow itself.
However, there are many cases where you might want to pass data into a form would never be provided by the responder directly:
Tracking information, such as unique identifiers, account IDs, etc.
Configuration variables, which enable or disable certain functionality.
If you're familiar with HTML form semantics, this is the equivalent of a
<input type="hidden"> field which is part of a form, but not provided by the user.
To enable this, you can create external answers within the Schema tab of your flow.
If a default value is set, if a value is not present when a flow is loaded, the default value will be used.
A common use for this is to define control variables, similar to constants, that govern what parts of a flow are enabled, or function as a setting parameter.