A category is a list of related possible job experiences that usually do not happen concurrently. (See Adding or Modifying Job Description Categories for further details.)
To modify a selection (say it's misspelled), click on the category in the menu on the left, or click on [Modify this list] at the bottom of the list in question in any of the job assignment description panes. The list of selections for that category will appear. Click on the offending item and you will see it appear in a text box to the left. Click on that text box and modify the entry. You will see the modification appear immediately in the list of selections. Press "Done" to return to what you were doing.
To add an entry, bring up the list of selections for the category in question and click on {New selection] at the bottom of the list. Type the name of the selection and you will see it appear, checked, in the selection list. Press "Done" to return to what you were doing.
If you do not wish to use a particular selection (perhaps you want to unclutter the list), simply uncheck it and it will not appear in any list of category selections. Press "Done" to return to what you were doing.
If you unchecked a selection that is used in any job assignments, it will disappear from those assignments. Rechecking the selection will cause it to reappear.