You shouldn't plan to land at destination with less than Reserve fuel (alternate + final reserve). However inflight circumstances can change...
Alternate fuel comprises of fuel following missed approach, climb, cruise, descent, arrival and approach to your alternate. Final reserve is 30mins holding fuel at 1500'AAL.
According to EU-OPS (which may be the documentation you refer to) IF onboard fuel is calculated to be LESS than the final reserve upon landing then a MAYDAY must be declared. My operator believes it to be prudent to declare a PAN call if it is believed that there is potential to land with LESS than the final reserve. This would be upgraded to a distress call should it actually become the case.
I believe ICAO use the phrase 'with reasonable certainty' when referring to inflight fuel re-planning and a subsequent landing when calculated fuel upon reaching the destination will be approaching reserve. If this is calculated to be the case then I believe the 'with reasonable certainty' reference could be applied. For this you need to be 'reasonably certain' that a landing can be made at the destination as opposed to heading straight for the alternate. Considerations relating to single or dual runway availability, traffic flow, weather etc would be considered. At this point you are making the conscious decision to encroach upon your alternate fuel in order to land at your destination. At no point should you ever plan to land with less than final reserve (hold fuel) when committing to a decision. To be honest I would never want things to get that close!!!