Using this attribute type means that every particle in the simulation will be assigned a random value between the specified minimum and maximum values.
For example if the Boid Separation attribute is given the values min=1.5 and max = 2.5, each particle in the simulation will be assigned a random value between 1.5 and 2.5 as their Boid Separation value.
The randomly chosen per-particle values are remembered for the duration of the simulation, however they are not saved with the scene and so if you close and re-open a scene you may find the simulation is not identical to the last time you ran it.
Also if either of the extents (min or max) are changed then all stored values will be immediately re-calculated. This is to ensure that the new extents are used by all particles. It is therefore not recommended that you change the extent values during a simulation (i.e. animating the extents) as this will lead to particles continually being re-assigned random values during the simulation.