pre tax money isn't taxed on contribution, is taxed on withdrawal. after tax is taxed in the year of contribution, isn't taxed on withdrawal. roth is 100% after tax, so you can access that category of funds selectively. in a traditional, if you have some after tax and some pre tax, any withdrawal is taxed on the % of each, you can't choose to withdraw one or the other selectively.
this is all afaik, i've never made any nondeductible contributions to a traditional, so i've never looked into the details much.