Initially, I thought they are safe ground clearance altitude. And they are not transition alt either
HI Danielle,
These are MORA - Minimum Off-Route Altitudes:
The MORA is the lowest altitude that provides guaranteed clearance from all terrain and obstacles within the grid square boundary. This is a crucial safety feature for pilots, especially when flying in instrument meteorological conditions (IFR) or as a general safety check during visual flight rules (VFR) flights, ensuring vertical separation from the ground.
See also Page 7 of https://download.navigraph.com/docs/guides/charts/introduction_to_navigraph_charts.pdf.
We are working on a project to make such information more immediately accessible to our users.
Cheers
Ian