This information is part of the Modelica Standard Library maintained by the Modelica Association.
If all arrows point in the same direction, a positive force results in a positive acceleration a, velocity v and position s.
For a force of 1 N and a mass of 1 kg this leads to
a = 1 m/s2 v = 1 m/s after 1 s (SlidingMass1.v) s = 0.5 m after 1 s (SlidingMass1.s)
The acceleration is not available for plotting.
System 1) and 2) are equivalent. It doesn't matter whether the force pushes at flange_a in system 1 or pulls at flange_b in system 2.
It is of course possible to ignore the arrows and connect the models in an arbitrary way. But then it is hard see in what direction the force acts.
In the third system the two arrows are opposed which means that the force acts in the opposite direction (in the same direction as in the two other examples).