Airline staff have a set number of people that they can list to receive sub-load tickets.

There does not have to be any proof given as to the relationship (I know of a guy bouncing back and forth between Hong Kong every 2 months and this is because another friend of mine offering this two him as he is single) but there is a limit as to how many people can be on your "list".

To all those people who think sub-load travel is the bees-knees, let me dispel this rumour. In very quiet times it is good (on xmas and NY day there are a lot of staff travelling), however at any other point it isn't. Try dashing from the checkin desks at CLK in HKG to the plane in under 10 minutes. Ground staff will hold you until all the seat allocations are done (including upgrades) and then at the last minute will release the seats to staff.

With regards to why people buy ID90/75 tickets in J and F, usually it is because it is easier to get in than economy on some routes. Also, a staff member with a J ticket will get on the plane before a staff member with a Y ticket even though both have the same date of joining.

Regarding cost, ozzie is correct. The fare is based on the full rack rate. To give you an idea, it used to cost $550 BNE-HKG in J on CX and about $1000 BNE-LHR in J on CX.

Hope this helps