This is scope of the variable, documentation on it can be found in the old doc: http://ez.no/ezpublish/documentation/reference/template_functions/variables/set_block
Also a lot of comments about it in the new doc:
http://ez.no/doc/ez_publish/technical_manual/3_9/reference/template_functions/variables/set#comment14285 http://ez.no/doc/ez_publish/technical_manual/3_9/reference/template_functions/variables/set_block#comment14300
root scope: $:variable global scope: $#variable
Namespaces in variables: with root scope: $YourNameSpace:variable
But it's not used to much any more other then in set-blocks when you want to set global variables like cache_ttl (view cache timeout) and persistant_variable ( a variable that is cache safe to send from node templates to pagelayout)
