It is long established that multiply-acting dislocations sources are not expected nor are they ever observed in stage II deformation of bulk materials . By analyzing large scale discrete dislocation dynamics simulations  of relaxed fcc microstructures  loaded under multislip conditions, dislocation multiplication is quantified in terms of source length and therefore source strength as well as nucleation rates. It is observed that individual nucleated dislocations (by cross-slip or glissile junctions ) only have a finite and local contribution to plastic deformation and no source-like object acts multiply.
On average the dislocation source length is correlated with the mean dislocation density, but the distribution of the identified sources varies considerably with the inhomogeneous local stress state. Unexpected small sources are identified and the reason for their activity is described. Nucleated dislocations do not glide very far before they themselves trigger a new nucleation event. This also raises the question when dislocation density transport is more relevant than a physical model for dislocation nucleation occurring in cascades.
The presented measures are useful for the development of a source model in dislocation density based continuum theories, since the three dimensional information as well as the statistics are studied in detail. Especially debunking the idea of source models with Frank-Read sources, which is used e.g. as a nucleation model in discrete dislocations plasticity modeling in 2D, which does not seem to capture wide ranges of material behavior or seem overly complicated.
 D. Kuhlmann-Wilsdorf, Chapter 59 The LES theory of solid plasticity, Dislocations ins Solids Vol. 11, Eds. F.R.N. Nabarro and M.S. Duesbery, Elsevier (2002) 211
 D. Weygand, L.H. Friedman, E. Van der Gieesen, A. Needleman, Model. Simul. Mater. Sci. Eng 10 (2002) 437
 C. Motz, D. Weygand, J. Senger, P. Gumbsch, Acta Mater. 57 (2009) 1744
 M. Stricker, D. Weygand, Acta Mater. 99 (2015) 130