org-export-select-tags

org-export-select-tags is a variable defined in `ox.el'.
Its value is

("export")

Documentation:
Tags that select a tree for export.

If any such tag is found in a buffer, all trees that do not carry
one of these tags will be ignored during export. Inside trees
that are selected like this, you can still deselect a subtree by
tagging it with one of the `org-export-exclude-tags'.

This option can also be set with the SELECT_TAGS keyword.

You can customize this variable.