To illustrate the correct Loong's answer more, let's see again example unsaturated fused polycyclic hydrocarbon azulene atom numbering1 (top left), and peripheral bond (edge) letter “numbering” for fusion nomenclature (top right).
Below is the skeleton of your product (without the two fused benzenes, before the dibenzo[…]… nomenclatoric operation), atom numbering2 (bottom left).
However, letter “numbering” of edges (bonds) is not used for polycyclic saturated hydrocarbon type nomenclature, as the names are constructed in a different way3 (also there's no single continuous edge loop defined). But if someone would try to do that similarly like for unsaturaded polycyclic ring numenclature, he/she would come with bond letter-ing, including those h,k (bottom right).
It's really strange that the very notable chemical seller uses this weird, wrongly constructed name as (the only) product name (webarchived) (or did you see it elsewhere?); see e.g. PubChem record for alternatives. (By the way, I think that a decent name, although maybe not preferred as well, is 3,4-(9,10-dihydroanthracene-9,10-diyl)oxolane-2,5-dione, maybe yet better with bridging nomenclature using prefixes like benzeno, furano, epipyrano, …: 11,15-dihydro-9,10-[3,4]furanoanthracene-12,14-dione (see e.g. examples in BB2013 P-220.127.116.11))
- Note that the non-peripheral, fusion atom numbers are appended with latin letters, but these are unrelated to edge (bond) letters.
- Note the different numbering for saturated polycyclic hydrocabons, starting at “fusion” (wrong term in this context), or bridges junction; also no letters added anywhere.
- The dibenzo derivative would be named as unsaturated pentacyclic hydrocarbon, pentacyclo[6.6.5.02,7.09,14.015,19]nonadeca‐2(7),3,5,9(14),10,12‐hexaene