The label can be any expression assigned to any object. It is stored as a character attribute. Inheritance is not implemented, so by any transformation of the object into another one, the label is lost.
References
S\'olymos P. (2008) mefa: an R package for handling and reporting count data. Community Ecology9, 125--127.
S\'olymos P. (2009) Processing ecological data in R with the mefa package. Journal of Statistical Software29(8), 1--28. tools:::Rd_expr_doi("10.18637/jss.v029.i08")