<!-- 5 minutes -->
<reserve reserve="PT5M">
  <rule>
    <!-- 10 minutes every 100 kilometres (100000 metres) -->
    <distanceBasedRule referenceDistanceMeters="100000" supplement="PT10M" />
  </rule>
</reserve>

The only extension we would require in our software would be to allow this element to be present in the rule element. However we need some clarification if this element should be used as I described. While I believe we have a legitimate use-case for it, it was also subsequently deleted from the upcoming 3.4 version, suggesting a deprecation path.

reserveTimeSeconds = (absReserveTimeSecondsPerDistance * distanceMetres) / 100000
                   = (600 * 50000) / 100000
                   = 300

• The formula above allows distances smaller than the reference distance, which is our interpretation of the current distanceBasedRule specification. The provided example is also one such case of this.
  
• The rule element has an informational nature only, detailing how the given time value was calculated. So any inaccuracies in the rule are mitigated by this; the specification states that the value to be considered (aka the "source of truth") is the reserve attribute of the reserve element:
  
  
  
  • rule: "A runtime reserve calculation rule can be provided for ducumentation purposed."
    
  • reserve: "The rule element can be used to further explain the value of this attribute, the field value however is the leading data here."
    
  • minimalTime: "Between two base itinerary points several runtime reserves can be specified independently from each other. The actual runtime of the train in a section is calculated by adding the minimum runtime to the sum of all supplements."