Spreading is a percentage factor that defines how a sound source will appear to spread out across speakers or virtual speakers. It is similar to Aperture in its focussing effect but will translate differently across certain channel based speaker arrangements according to how many speakers are involved.
This parameter is only available to a source, if the room it is simulated in has been specified to be using the K Nearest Neighbour panning type (see section 5.64). It sets a maximum limit to the number of speakers that the algorithm can use as neighbours in its search for speakers to activate in relation to a virtual source. On a 10 speaker setup, 1-10 % will be the closest speaker to source. 11%-20% will be 2 and so forth.