How come in naming OCDD no numbers are used to specify the positions but 2,3,7,8 -TCDD does?

in OCDD all of the available eight positions are filled (Octa Chlorinated DD) - no need to say 1,2,3,4,5,6,7,8 - OCDD.





in TCDD only four of the eight available positions are filled, so their positions would be named.





(TC stands for Tetra Chlorinated meaning Four Chlorinated)