Currently each unique combination of values of vars constitutes a subset. In the future, specifying shingles for numeric conditioning variables will be implemented.
References
http://tessera.io
http://onlinelibrary.wiley.com/doi/10.1002/sta4.7/full{Guha, S., Hafen, R., Rounds, J., Xia, J., Li, J., Xi, B., & Cleveland, W. S. (2012). Large complex data: divide and recombine (D&R) with RHIPE.Stat, 1(1), 53-67.