Within the scope of a tag library there are a number of pre-defined variables including:
actionName - The currently executing action name
controllerName - The currently executing controller name
flash - The flash object
grailsApplication - The GrailsApplication instance
out - The response writer for writing to the output stream
pageScope - A reference to the pageScope object used for GSP rendering (i.e. the binding)
params - The params object for retrieving request parameters
pluginContextPath - The context path to the plugin that contains the tag library
request - The HttpServletRequest instance
response - The HttpServletResponse instance
servletContext - The javax.servlet.ServletContext instance
session - The HttpSession instance