From the second paper[0]:

'In order to provide more information about event severity within the S1 designation, S1 severity events have been separated into two columns in Table 1 based on whether each event is of sufficient severity to result in actual or simulated airbag deployment for any involved vehicle. Of the eight airbag-deployment-level S1 events, five are simulated events with expected airbag deployment, two were actual events involving deployment of only another vehicle’s frontal airbags, and one actual event involved deployment of another vehicle’s frontal airbags and the Waymo vehicle’s side airbags. There were no actual or predicted S2 or S3 events'

[0] https://storage.googleapis.com/sdc-prod/v1/safety-report/Way...