Yes, Mp3tag provides content variables written as of format %name% as information fields.
Such an info-field is no tag-field at all, therefore the content of an info-field cannot be changed by the customer.
It is somewhat confusing, because both, the content of a tag-field, and the content of an info-field, is queried by the same content operator clamb %...%.
But we are lucky, there is a small help to distinguish a tag-field from an info-field:
the leading underline character "" determines an info-field.
In case of the pseudo tag-fields _FILENAME and _DIRECTORY and their content %_FILENAME% and %_DIRECTORY%, which can be manipulated by Mp3tag, the confusion is perfect, because there also exist info-fields with the same visual incarnation %_filename% and %_directory%.