In a Vox column, Matthias Schlegl, Christoph Trebesch, and Mark Wright document an implicit seniority structure of external sovereign debt: IMF > Multinational > Bonds > Bilateral > Banks > Trade Credit (see the figure).

They argue that Greece’s recent default on the IMF constitutes an outlier.

… Greece in 2015 is clearly an outlier case, having defaulted on the most senior creditor (the IMF), while continuing to service historically more junior creditors. The evidence also suggests that the Eurozone rescue loans, which are essentially bilateral (government-to-government) credit, are likely to be a junior creditor class going forward. The evidence also rationalises why Greece may have an interest in exchanging the debt it owes to the IMF and the ECB into loans to the European Stability Mechanism, which is likely to be junior debt in the future, as discussed in the run-up to the July Eurozone summits. Policymakers should be aware of the associated changes in seniority and repayment incentives.