Sometimes it may be necessary to export content in sub-folders. For example, CSS files might go in a /css folder. In such cases, place the files in container off the root (or nest as many 'dummy' containers as needed).
Source container has no $Text
If the container has no $Text content, then a folder is exported with a name based on $Name and containing any exportable child notes.
A different folder name from $Name
What if the container name isn't correct? It may be the case is wrong or the necessary folder name needs to differ from the source container. If so, set $HTMLExportFileName to the appropriate name using the case desired.
$Text present but no export desired
What is there is text in the container note but it isn't to be exported, e.g. because it is instructions about what lives in the container? In this case set $HTMLDontExport to true or open the container's HTML view and un-tick the top left box. $HTMLDontExport is a little confusing as the value is the opposite of the HTML view tick-box, i.e. ticked equates to false - this is the reverse of the way most booleans work.
Don't edit the export extension!
Regardless of the above, even if the container is exported only as a folder don't be tempted delete the export extension ($HTMLExportExtension) or you get an unexpected result - the folder is created but child notes are created as its siblings. There's an arcane reason for this logic, but remember, TB wants a file extension for a container even if it is only ever exported as a folder.
Containers children are only boilerplate/includes
A reverse scenario occurs where you want to use notes in a container but not have the notes or the container export, such as when the child notes are only ever used as includes in other exported files. In this case, open the HTML View of the container and un-tick the first two boxed top left - for 'Export' and 'Export Children' (setting $HTMLDontExport and $HTMLExportChildren respectively). Meanwhile, ensure the child notes - the include content, have their 'Export' box ticked in HTML view. This is slightly counter-intuitive at first meeting, but to be exported as an include, the source of the include content must be an exportable note. By placing the note in a parent container that is set to not export its children, the correct conditions are met.