The options in the Generation and Overlap Mode panel control the relative position among objects (or with the structure currently in memory).

Objects may touch (but not overlap) when Prohibit Objects Overlap is selected. Objects may not touch when Use Isolation Distance is selected, and a positive value is entered. Then, all objects have at least this preset distance. The choice of Use Isolation Distance may lead to long computation times for large structures. In contrast to Create Grains, negative values for the Isolation Distance are not allowed.