This is more covered by Responsible Lending which essentially says you cant lend people more than they can afford. So in essence each lenders look at your salary, take off what they calculate as reasonable expenses (varies from lender to lender but is based on what you tell them adjusted by some figure they have on how much people really spend - this stops those who say they spend nothing). This leaves money available to pay loans.I imagine total combined limits vs annual income/available discretionary spend is something the banks have always looked at. Any idea how important such a factor is and how charge cards with no set limit might fit into the mix?
In terms of this they have to take into account existing lending, the trouble here is they only know limits (from the Bureau) or what you tell them. Each lender has their own way they assess how much you owe, for a charge card hard to tell, its probably just a reasonable limit of what you could spend (and taking into account the limits aren't really limitless - Amex wants to get its money back too, and as a charge card you need to pay back every month).