Warning message when exporting to Exodus

I am receiving the following message when exporting an exodus file:

Using ExodusII API version 4.98 and DB version 4.98 to export “…/filename.e”.
WARNING: The model contains 290526 nodes (and their attached elements)
that will not be written to the genesis file.
This may indicate a problem in element block assignments.

What exactly is the message trying to tell me?

That usually means you have missed assigning some part of the mesh to a block. Check to see that all elements have a block assignment. You can draw individual blocks by picking their block ID from the tree view. If you only want to export a portion of the mesh, that message may be OK.