The buffer-tabs in XEmacs have an automatic filtering set up to display groups of similar file-extensions. I believe you can also filter on other attributes that are discoverable about the buffers in the buffer-list as well.
It is useful when you have many files open, on a small monitor, but gets in the way other times, so you can set the filtering off by, well, here is an excerpt from the XEmacs manual:
"This behavior can be altered by customizing buffers-tab-filter-functions'. Setting this variable to
nil' forces display of all buffers, up to buffers-tab-max-size' (also customizable). More complex behavior may be available in 3rd party libraries. These, and some more rarely customized options, are in the
buffers-tab' Customize group."