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A general description is here.
The [[ subsetting operator now has an argument 'exact' that allows programmers to disable partial matching (which will in due course become the default). The default value is exact=NA which causes a warning to be issued when partial matching occurs. When exact = TRUE, no partial matching will be performed. When exact = FALSE, partial matching can occur and no warning will be issued. This patch was written by Seth Falcon.
It is planned that [[exact=TRUE]] will become the default in R 2.7.0.