It seems to me we're all taking the same language. The original post was about...
><snip> actually there are only three forces. Weight, thrust and
>total reaction (TR). This TR can be broken down, by the
>scientists, to show a horizontal vector (not a force) which we
>call drag - and a vertical vector which we call lift.
Actually their are an infinite number of forces acting on a plane that sum to zero in S&L constant velocity fight - but we prefer to resolve them (break them down into) components that act in specific directions.
You could say...
The sum of all forces = 0
or
W + T + D + L = 0
or
W + T + L + TR = 0
or
W + everything else = 0
It's all means the same thing.
The important bit is that the sum = zero. If it doesn't then there
is a residual force acting on the plane that will cause it to depart
from S&L constant velocity flight.