What it does
This tool replaces the entries of a defined column with entries given by a replacement file. For example the replacement file holds the information of the naming scheme of ensembl annotated chromosomes in the frist column and in the second the UCSC annotation. A file which is having information about chromosomes in ensembl notation in column x can now be converted to a file which holds the same information but in UCSC annotation.
A useful repository for ensembl and UCSC chromosomes mapping is: https://github.com/dpryan79/ChromosomeMappings