What happens when I set the option to exclude a category from manual journal entry?

The category won't appear in the list of available categories on the Create Journal page and in the Create Journal spreadsheet. You can still use the category for journals imported from Oracle subledgers or third-party sources.

And you can still view journals with that category on the Manage Journals page.

Note: After you select the Exclude from Manual Journal Entry option on the Manage Journal Categories page, you may need to sign out of the application and sign back in for the change to take effect.