How to understand "Stage length, for payload calculations includes 85% wind"?
The document AEA Requirements Long Range Aircraft is related to airline fleet planning, and the calculation of the fuel needs. Note that it is dated Dec. 1987. More on airline fleet planning.

85% wind

For this planning the document from the airlines association recommends to assume the wind that has a probability to occur of 85%. That means: assume you will find this wind on your route 85% of the time. The wind force itself is determined talking into account statistics for the regions overflown.

Stage

The flight profile is provided on the document itself, defining "stage" as start of climb to end of descent, excluding initial climb:



Planning profitability

Knowing how much fuel will be required and by deduction how much payload can be transported allow airlines to estimate the airline costs and revenues on a given route with a given aircraft. This is important for planning aircraft acquisition and replacement, as well as the route catalog.