The border between the “upper” and the “lower” levels show the audial (in particular, musical) and the literary codes.
Karpukhina, V.N. 2017, 'SEMIOTIC CODES OF SPACE IN THE TEXTS BY MIKHAIL BULGAKOV: AN ANALYTICAL REVIEW', Russian Linguistic Bulletinhttp://rulb.org/wp-content/uploads/wpem/pdf_compilations/1(9)/1(9).pdf#page=68. Retrieved from DOAJ CC BY 4.0 (https://creativecommons.org/licenses/by-sa/4.0/legalcode)
One can single out the following types of sources: verbal, pictorial, audial, behavioral and conventional ones.
Angela Papazova 2017, 'The classification of historical sources on Jesuit practices in the East Slavic regionin the late XVI - early XVII centuries', Cхідhttp://skhid.kubg.edu.ua/article/view/91526. Retrieved from DOAJ CC BY 4.0 (https://creativecommons.org/licenses/by-sa/4.0/legalcode)