I enountered this concept and thought it was nifty but have not time to do anything other than note it here.

(Le and Clarke 2017) summarise

For the sake of completeness, we recall that Bernardo and Smith (2000) define M-closed problems as those for which a true model can be identified and written down but is one amongst finitely many models from which an analyst has to choose. By contrast, M-complete problems are those in which a true model (sometimes called a belief model) exists but is inaccessible in the sense that even though it can be conceptualized it cannot be written down or at least cannot be used directly. Effectively this means that other surrogate models must be identified and used for inferential purposes. M-open problems according to Bernardo and Smith (2000) are those problems where a true model exists but cannot be specified at all.

Bernardo, José M., and Adrian F. M. Smith. 2000. *Bayesian Theory*. 1 edition. Chichester: Wiley.

Le, Tri, and Bertrand Clarke. 2017. “A Bayes Interpretation of Stacking for $\mathcal{}M{}$-Complete and $\mathcal{}M{}$-Open Settings.” *Bayesian Analysis* 12 (3): 807–29. https://doi.org/10.1214/16-BA1023.