Digital Connect Check-In supports check in of staff (airline employee) passengers travelling on non-revenue tickets of both types, space available and positive space.  
There is no specific handling of this type of passengers in Digital Connect Check-In application. The verification of seniority date and staff priority is being done by Airport Check-In System based on the data in Passenger Name Record (PNR)

Digital Connect Check-In returns basic information about the non-revenue passenger and segment in /dcci/passenger/details 

Example: 

{
  "passengerSegment": [
    {
      "id": "p01.01.s1",
      "passengerFlight": [
        {
          "id": "p01.01.s1.f1",
          "flightRefs": [
            "f1"
          ],
          "checkedIn": false,
          "status": "NOT_CHECKED_IN"
        }
      ],
      "nonRevenuePassengerInfo": {
        "priorityCode": "C9",
        "seniorityDate": "2010-01-01"
      }
    }
  ]
}

{
  "segment": [
    {
      "id": "s1",
      "status": {
        "value": "NON_REVENUE",
        "code": "MM"
      },
      "flightDetail": [
        {
          "id": "f1"
        }
      ],
      "seatStatus": "STANDBY",
      "number": "3"
    }
  ]
}

Error Handling:

When the segment is booked as non-revenue but there is no priorityCode information, DCCI will return the following error: 
{
  "results": [
    {
      "designator": {
        "value": "p01.01.s1",
        "context": "PASSENGER_SEGMENT"
      },
      "status": [
        {
          "message": "Expected data for non-revenue passenger is not available.",
          "code": "DATA_MISSING_FOR_NON_REVENUE_PASSENGER",
          "type": "WARNING"
        },
        {
          "type": "SUCCESS"
        }
      ]
    }]
}

The following keys are introduced to support Staff Travel:  

• s4ci.checkin.eligibilitiesBlockingCheckin 
• s4ci.checkin.eligibility.nonRevenueWindow.enabled 
• s4ci.checkin.nrpsSeatSelectionEnabled 
• s4ci.feature.seats.nonRevenueTypesAllowedForPaidSeats 
• se.adapter.seatmap.esm.blockSeat.nonRevenueGetSeatMap