There are lots of knowledgeable folks here that can go into the weeds but essentially it's this:
An exportable elephant counts against the CITES quota for that country. Lots of science goes into determining the quota. Exportable does not mean Importable. You'll need to check your country's import laws to see if you're allowed to bring it in (ex. In USA the USFWS places additional restrictions and permitting on top of CITES).
A non exportable elephant is generally determined by the country's government to be legal to hunt, but not to count against the CITES quota. These might include problem animals, community use, etc. Confirm with the outfitter what exactly qualifies the elephant as non exportable to ensure everything is legit. Example- forest elephant are legal to be hunted in Cameroon, but are not legal to export.