The source node in most cases contains the sample message as written by the developer. If it looks like a dot-delimitted string such as "form.label.firstname", then the developer has not provided a default message.

Maximum {{ limit }} chars Maximum {{ limit }} chars key: Maximum {{ limit }} chars Please enter minimum {{ limit }} chars Please enter minimum {{ limit }} chars key: Please enter minimum {{ limit }} chars This guid already exists This guid already exists key: This guid already exists You must choose one payment method You must choose one payment method key: You must choose one payment method You must choose one shipping method You must choose one shipping method key: You must choose one shipping method