A debt exists whenever someone owes money to someone else. The debt is an asset held by the person to whom the money is owed, the creditor. The repayment of the debt is a disposal of the asset by the creditor for capital gains purposes, TCGA92/S251 (2). Alternatively, the creditor may dispose of the debt by selling or transferring it to another person.
Although all debts are assets, some debts do not give rise to chargeable gains on their disposal. For example, debts held by the original creditor will not normally give rise to chargeable gains, unless they are debts on a security. Further advice on what constitutes a debt on a security, and on the tax treatment of debts generally, is given at CG53400+.