if i'm going to check data user submit to make sure its value is ok,which code should i modify?
Not quite sure what do you mean, but it all could be done at the add-on's level. You may verify the posted data in bb_plugins.php on the condition like
and then you could supply a default value for the $_POST['occupation']
there, if someone is trying to post a fake/wrong value and so on.
I.e. in this bb_plugins.php you could catch the incoming parameters, verify, parse and re-assign them, before the core script executes.