The background and history of all wage/salary supplements will vary according to the industrial history of the particular contract.
The trend in these times is to pay flight crew (and many other staff) a 'retainer' BASIC salary (mine is about half of what I actually take home), and above and beyond this, pay for productivity. The airline industry is not unique in this respect.
The number of sectors flown, the hours flown, the distance flown, etc. are all fairly good indicators of productivity.
As
knackeredII indicates "
It's not as good as it appears. It's another scheme for employers to pay you less, in a lot of cases substantially less, when you are on leave, standby etc. ".
Ditto to that oh knackered one
Regards,
Old Smokey