Liabilities: law relating to debts: right to reimbursement
The right to reimbursement is most likely to arise in connection with a guarantee debt (IHTM28354). But it may also arise in relation to, for example, a lessee who has assigned a lease - in which case the lessee would be entitled to reimbursement from the assignee. And in Re Latham  Ch 616 a debt, being Estate Duty payable on the death of a first life tenant that remained unpaid, was held not to be a permissible deduction from the estate of a second life tenant. This was on the basis that the second life tenant’s free estate had a right to reimbursement from the capital of the trust fund of which he was tenant for life.
The right to reimbursement can also arise in connection with lifetime transfers. If the deceased makes an immediately chargeable transfer which gives rise to a tax liability and the tax payable is deducted from the estate then you should call for the lifetime file and consult with Technical.