I have two boxes, $A$ and $B$ that exchange mass.
I will call the flux of mass between $A$ and $B$ $ab$; $ba$ is the reverse.
$X = ab -ba$
I am trying to understand the following statement:
$dA/dt$, $dB/dt$, $ab$, and $ba$ are positive definite since they represent a one-way process. Whereas $X$ is a vector with a sign that implies the direction of transfer.
The origin of my confusion is that when the terms $X$, $ab$, and $ba$ are plotted together over time, $ba$ is plotted with negative values. I imagine that this is justified by rewriting the equation as $X=ab + (-ba)$.
I have never seen the term "positive definite".
Here is a diagram of the problem: