It is by and large only un-secured debt which creditors will consider settlement propositions for. Un-secured debts are those that don't require any collateral (security); such as credit cards and service bills for instance.
Some examples of secured types of debt would include things like car or mortgage loans, in which case the automobile or house would be used for security/collateral respectively.
Why only unsecured debt?
When a borrower doesn't pay their bills on an unsecured debt, there's nothing for the lender to repossess to make up for the loss.
That puts them in the often frustrating position of trying to collect the debt with paperwork and phone calls. When that
doesn't work, they don't have much recourse to recover their loss, and would normally prefer a partial payment than nothing.
In contrast, with a secured loan the lender would normally recover more by repossessing the collateral that was put up, so they aren't really interested in a settlement proposal when it's not in their financial best interest.
These are some conventional un-secured debts that are often negotiated:
Settlement terms can vary a lot depending on the debtors particular condition and who you're dealing with, and creditors are not obligated to accept debt settlement proposals at all. It is more common for lenders to accept negotiations when they are in fear of the debtor going bankrupt, or other situations in which a partial settlement is likely to be more than what they would receive otherwise.