Your eligibility score is calculated by HD Decisions Ltd, a company we've partnered with in Credit Club (and in our eligibility calculators). HD has worked closely with lenders to learn details of the types of customer they're looking for to lend to for credit cards and loans.
HD Decisions looks at details of your credit report(s) as well as the information you've given us (eg income) to see how well you match lenders' criteria for a range of cards and loans.
If you're a good match for a particular lender's criteria, you'll get a high eligibility score for their products. Similarly, if you've none of the attributes a lender is looking for, you'll get a low or zero score, as that lender would be unlikely to lend to you if you've applied.