Y, J and F are their own fare buckets, but also used as umbrella terms for Economy, Business and First. When I said "In Y EK will Show availability as high as 9" I meant in all Economy fare buckets, which are Y down to X. On PC now so can copy the loads of 415:

F4 A4 J7 C7 I7 O0 H0 P0 Y5 E5 R5 W5M5 B5 U0 K0 Q0 L0 T0 V0 X0

As you can see another seat has been sold since the earlier check and now down to 5.

From left to right it pretty much follows the F, J and Y pattern:

F & A = First Class (F)
J, C, I, O, H & P = Business Class (J)
Y to X = Economy Class (Y)

Generally speaking the further left within a cabin the fare bucket is the higher the fare, e.g. Y, E & F are Flex Plus fares while X is a Special fare.

The flight can be oversold because Emirates will sell the total number of seats on a flight. Meaning if the combined unsold seats in J and F are 20 they will oversell Y by around 20. It's not always exact, sometimes they over-oversell and this results in IDBs (Involuntary Denied Boarding). On EK this can be quite lucrative as they will rebook you on a later flight in addition to giving you a free return ticket which you can use within the next 12 months, so if you have some flexibility and travel more than once a year this can be very useful.