Basically if Y is full who gets J first out of the following:
40Y/J1789 or 41Y/J1594
Assuming both have paid for a J upgrade.
If there is only 1 seat left in J then the 40Y/J1789 would get it as they have the higher onload to get onto the acft first. If there were 2 seats left in J then they both get it.
If there was 1 Y and 1 J left then 40Y would be in Y and 41Y would go to J
Hope it makes some sense. Staff tvl categories are an endless cause of angst