In order to restrict authors to post only to the selected list of categories use posts/pages edit restrictions add-on:
– activate add-on at Settings->User Role Editor->Additional Modules;
– go to Users->User Role Editor;
– select ‘Author’ role;
– grant ‘create_posts’ capability to this role;
– click ‘Posts Edit’;
– select “Block” or “Allow” action;
– input a list of digital category IDs to which you wish to block or allow access;
– save your changes.