%PUBURL% expands to http://biostat1478.dhcp.mc.vanderbilt.edu/foswiki/pub
%PUBURL{"icon_plus.png"}% expands to =%PUBURL{"icon_plus.png"}%
%PUBURL{web="System"}% expands to http://biostat1478.dhcp.mc.vanderbilt.edu/foswiki/pub/System
%PUBURL{topic="System.MainFeatures"}% expands to http://biostat1478.dhcp.mc.vanderbilt.edu/foswiki/pub/System/MainFeatures
%PUBURL{web="System" topic="MainFeatures"}% expands to http://biostat1478.dhcp.mc.vanderbilt.edu/foswiki/pub/System/MainFeatures
%PUBURL{topic="System.MainFeatures"}% expands to http://biostat1478.dhcp.mc.vanderbilt.edu/foswiki/pub/System/MainFeatures
%PUBURL{topic="System.MainFeatures" "icon_plus.png"}% expands to http://biostat1478.dhcp.mc.vanderbilt.edu/foswiki/pub/System/MainFeatures/icon_plus.png
topic_version and attachment_version parameters. These can be used with advanced store implementations to select specific attachment versions. However simple file-based stores do not normally support them. 
The 'old' way of building URLs using PUBURL involved concatenating the web and topic names to the PUBURL e.g. %PUBURL%/Main/SystemFeatures. This practice is strongly discouraged, as it does not correctly handle encoding of the parts of the URL. At the first opportunity you should replace all such URLs with the equivalent %PUBURL%{topic="System.MainFeatures"}%, which will handle URL encoding for you.
ATTACHURL provides a shorter way to refer to the attachments on the current topic.