The man page can be found here:
http://pic.dhe.ibm.com/infocenter/clmhelp/v3r0m1/index.jsp?topic=%2Fcom.ibm.team.concert.dotnet.doc%2Ftopics%2Fr_comp_arguments.html

It contains the following information:
Compare arguments
The compare arguments defines the specifications including file name, file path, and remote file name for the files to be compared. Each of the listed compare tool has a set of arguments. If you use a compare tool that is not listed, you have to use new arguments. The format of the new arguments must be as "${newArgument}".
Arguments

${file1Path}
Path to the local file
${file1Title}
Title for the local file in the compare tool
${file2Path}
Path to the repository file
${file2Title}
Title for the repository file in the compare tool
${ancestorFilePath}
Path to the previous version of the repository file
${ancestorFileTitle}
Title for the previous version of the repository file in the compare tool
${mergeFilePath}
Path to the merged file
${mergeFileTitle}
Title for the merged file in the compare tool

Is there a man page that describes the different variables used in the external compare tool settings.

Setting it up is described in Setting up external compare tools
http://pic.dhe.ibm.com/infocenter/clmhelp/v3r0m1/index.jsp?topic=%2Fcom.ibm.team.scm.doc%2Ftopics%2Ftexternalcomparetool.html

But searching for the various arguments on the web or in the admin guide does not return any hits.