This option enables you to specify some custom data that will be placed in a session called "mc_custom_data" this session data can then be picked up by for example the custom Authenticator class.
Example of usage in init call on page level:
mcFileManager.init({
custom_data : "some custom data"
});
Example of usage in mcFileManager.open call
<a href="javascript:mcFileManager.open('example1','url','','',{custom_data : "some custom data"});">[Browse]</a>
Example of usage in a TinyMCE init call
tinyMCE.init({
mode : "textareas",
theme : "advanced",
plugins : "advimage,advlink,filemanager",
filemanager_path : "testcases/multiple_rootpath_files/root2"
});