Spirent imports these routes organized by route blocks.
It could be looked at as the route blocks are the number of rows in the file and the routes per block is like the number of columns. These two values determine the area of the rectangle, but the routes form the route file may fall out of this area due to too many rows or too many columns. So setting the max values for route blocks and routers per block is not clearly defined and needs some educated guess when setting these values but a summary screen will show you how many routes got imported from the file.
Spirent is targeting improvement on this in future releases.