The bare bones MP3 file reader supplied in tuneR reads the entire file in. When the user installs the third party software mp3splt and libmp3splt, this variant will allow from and to to be specified, and mp3splt will attempt to read in the MP3 segment without first decoding the file. Because mp3splt will cut the MP3 file at frame boundaries the from and to arguments are necessarily only guiding values; actual values may differ. Supplemental mp3splt installation instructions are provided in the document "Installing_mp3splt.pdf", available the monitoR website http://www.uvm.edu/rsenr/vtcfwru/R/?Page=monitoR/monitoR.htm.
References
mp3splt is documented at http://mp3splt.sourceforge.net/mp3splt_page/home.php.