contr.dummy is standard dummy-coding, contr.metric has the
same effect like as.numeric (makes sense of course only for ordered
variables). contr.ordinal computes contrasts for ordinal variables.
References
Hechenbichler K. and Schliep K.P. (2004) Weighted
k-Nearest-Neighbor Techniques and Ordinal Classification, Discussion Paper
399, SFB 386, Ludwig-Maximilians University Munich
(tools:::Rd_expr_doi("10.5282/ubm/epub.1769"))