Nspans*Lspan/Nsteps defines the

considered step-size

Please check Fig. 6: Estimated maximum reach as a function of the DBP step-size, for CDE, SSFM, W-SSFM and W-VSNE methods.

There's a tradeoff between step size and complexity

http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=7364164&newsearch=true&queryText=Low%20Complexity%20Advanced%20DBP%20Algorithms%20for%20Ultra-Long-Haul%20400%20G%20Transmission%20Systems