It is actually very simple.
On the ground take-off is optional. All
planned landing airports and alternates must meet dispatch landing distance.
Once airborne, landing is mandatory. All landings are en-route/advisory data.
From an IFALPA document that discusses the differences between certified and advisory.
Comparing Certified to Advisory data is, as the saying goes, like “comparing apples with oranges”. Certified data (that is factored dry test landing distance, with no reverse thrust, multiplied by 1.67 plus an additional .15 for wet runways) is used as a ight planning tool. Certified data allows you to determine the maximum takeoff weight which will allow the aircraft to land at the destination or alternate airport. FAA Advisory data is non-factored and assumes the use of reverse thrust in addition to all the conditions met or adjusted as referenced from the chart at the bottom of the PI page. Advisory data is provided to meet operational needs on varying runway conditions with the expectation that crews will assess landing performance based on actual weather and runway conditions at the time of arrival as opposed to those prevailing at the time of dispatch, when Certified data is used.
http://www.ifalpa.org/downloads/Level1/Briefing%20Leaflets/Aircraft%20Design%20&%20Operation/12ADOBL03%20-%20Certified%20vs%20advisory%20data%20on%20Boeing%20aircraft .pdf