When starting each sub batch, the first point value is chosen randomly.
When a bad point is computed, a new point is also chosen randomly.
This field allows you to specify the range in the x and y dimensions that the new point is chosen from in both cases.
For example, the default value of 1 means random points will have an x coordinate between -1 and 1, and a y coordinate between -1 and 1.
In practice, this has no noticeable effect on most images and therefore will almost never need to be adjusted. However it has a very noticeable effect on others.
One scenario in which adjusting this field is useful, is when pasting in parameters from Chaotica. They will usually look slightly different, but can be made mostly similar by adjusting this field along with the fuse count.