The internal_rating identifies the different categorizes for exposures that have no external ratings. Financial organizations can assign internal ratings to rate and categorize these exposures to get different risk weight assignments.
For corporate unrated exposures, firms may categorize as “investment grade”, “non-investment grade” or non-category. Reference OSFI, Chapter 4, P62-


Referencing Basel 20.46

An “investment grade” corporate is a corporate entity that has adequate capacity to meet its financial commitments in a timely manner and its ability to do so is assessed to be robust against adverse changes in the economic cycle and business conditions. When making this determination, the bank should assess the corporate entity against the investment grade definition taking into account the complexity of its business model, performance against industry and peers, and risks posed by the entity’s operating environment. Moreover, the corporate entity (or its parent company) must have securities outstanding on a recognised securities exchange.


See investment above.