In [1] and [2] the authors claim that the boundary states (not just the Ishibashi states) of a Kazama-Suzuki model are labelled in the same way as the primary states of the model, so that the boundary states can be seen as a subset of the set of primary states (which are related to the irreducible HW representations of $Vir$). However, they do not say where this result comes from.
Request: I would like to know of an article or textbook which discusses/proves this result. Alternatively, a clear explanation would be fine, although I would prefer if it were supported by references.
[1]: https://arxiv.org/abs/hep-th/0112221 Nozaki, M. (2002). Comments on d-branes in kazama-suzuki models and landau-ginzburg theories. Journal of High Energy Physics, 2002(03):027.
[2]: https://arxiv.org/abs/1005.2117 Behr, N. and Fredenhagen, S. (2010). D- branes and matrix factorisations in supersymmetric coset models. Journal of High Energy Physics, 2010(11):136.