[O] CUSTOM_ID only works when entire file is exported?

[Mark S]

I'm using C-c e b to do an HTML export.

It appears that CUSTOM_ID only gets assigned when the entire file (not  
just the tagged sub-heading/tree) gets exported. The same goes for the  
HTML_CONTAINER_CLASS variable, only its worse because it will sometimes  
prevent export altogether with a "Before first headline at position xx in  
buffer org-mode-tmp" message, which I have no idea what it means.

Is there anyway to export just one sub-tree (or heading group) with an  
assigned custom_id?

Thanks,
Mark