S.R.
Any debt on your credit report that is 90 days delinquent, in a collection status, charge off status. settled for less than full amount, or paid in full over 90 days + delinquent will all affect your score the same way. If you are already seriously delinquent then your score won't be any worse off by settling and you'll pay less out of pocket. The way a settled or paid account helps is because it reports as paid and not that there is a past due amount. The credit bureaus lower your score for every account with a past due amount owing as well as a delinquency status (30, 60, 90, charge off, etc). In essence your score is getting lowered twice, once for past due and again for delinquent. If you settle or pay in full then you are only getting hit for the delinquent status. All trades are supposed to fall off your credit 7 yrs after the date of the last payment.
As others have said, when you settle the bank is "forgiving" any unpaid debts but they do have the right to send you a 1099c at the end of the tax year up to 3 years after the last payment from you. IRS always gets their money. If you owe 5k and settle for 2k, you will have to claim 3k in income when you get the 1099c because it was money you spent and since you aren't paying it back it is considered income in the IRS's eyes. This could be for anything from a credit card to a deficiency balance owed on a car that was repo'd and then sold for less than what you owe.
The older derogatory or delinquent credit is on your credit report, the less impact it has on your score. A 30 day late this month will impact you hugely where as if it was over 4 years ago, it will be a minimal impact. The important thing is to clear up and pay off old debts and not go delinquent again. As long as you stay current going forward that will help your score too. How you pay accounts for 35% of your score. How much revolving credit you use accounts for 30% of your score. For those that might not know, a HELOC is included in your revolving account ratios even though it is secured by your house. If you have a maxed out HELOC (home equity line of credit) even with perfect credit your score can drop from the mid 700's to the low 600's. You never want to get a HELOC with the intensions of maxing it out.
If you are current or less than 90 days delinquent then I would call your creditor and try to negotiate a lower rate and reduction in fees and get on a payment plan. If the rep won't help you, escalate it until you get someone that will. They want your money, not everyone can pay cash out of pocket. If they could they wouldn't be in a collection/delinquent status. I know someone that got their capital 1 cards down to 0% interest and no fees and their BofA card down to 6% and no fees. Good luck.