That setting is a filter for the vectors (velocity and spin) the Slow operator operates on.
Take for example velocity. If a particle is moving from left to right, it's velocity vector might be something like [1,0,0]. If Slow is set to "values greater than zero", and the amount is set to 50%, after 1 time step the Slow operator will change the velocity to [0.5, 0, 0] (vel.x * 50%, vel.y * 50%, vel.z * 50%). However, if the velocity is [-1, 0, 0] (ie, moving right to left), then after 1 time step the velocity will remain the same if Slow is set to affect values greater than zero, because vel.x (-1) is less than zero and so will be ignored by the Slow operator.
An example usage of this is if you have some particle that fly up in the air too high...instead of increasing the value of gravity or damping their overall velocity, you could simply apply some Slow to their velocity z-axis and set the mode to "values greater than zero". With that setup, damping will be applied as they move upwards, but not downwards, thereby allowing for greater control over their motion without changing other simulation parameters like the strength of gravity of their initial rate of acceleration.
It doesn't make sense to have the default mode be anything other than "all values", though. The "greater than" and "less than" modes are only useful in some situations.
Take for example velocity. If a particle is moving from left to right, it's velocity vector might be something like [1,0,0]. If Slow is set to "values greater than zero", and the amount is set to 50%, after 1 time step the Slow operator will change the velocity to [0.5, 0, 0] (vel.x * 50%, vel.y * 50%, vel.z * 50%). However, if the velocity is [-1, 0, 0] (ie, moving right to left), then after 1 time step the velocity will remain the same if Slow is set to affect values greater than zero, because vel.x (-1) is less than zero and so will be ignored by the Slow operator.
An example usage of this is if you have some particle that fly up in the air too high...instead of increasing the value of gravity or damping their overall velocity, you could simply apply some Slow to their velocity z-axis and set the mode to "values greater than zero". With that setup, damping will be applied as they move upwards, but not downwards, thereby allowing for greater control over their motion without changing other simulation parameters like the strength of gravity of their initial rate of acceleration.
It doesn't make sense to have the default mode be anything other than "all values", though. The "greater than" and "less than" modes are only useful in some situations.