I can confirm they are indeed a cake as we make the flour for Jaffa Cakes (sole European supplier at that).
JL9 is the code & its grist is indeed that of a cake flour. Its called JL9 as that was the trial code, we went up to 15 before they decided that No 9 was the best 😕
The criteria for the flour is based around viscosity when mixed in very accurate amounts with a very accurate amount of water & then mixed in a Kenwood mixer for an exact amount of time, this is then tested in a extremely accurate machine or viscosity!
Well you did ask 😉