It can be a bit confusing...
The lower box contains the items that are attached to the post that you are editing. You can delete those items by putting your cursor on the item which will make a ? and X appear in the upper right corner of the item. Click the X to delete the item.
The upper box contains the items that are attached to other posts (not the one that you are editing). You need to be editing the post that an item is attached to in order to delete it. Fortunately you can go directly to the post that an item is attached to by doing the following. Put your cursor on the item and a ? will appear in the upper right corner of the item. Click the ? and a box will appear showing where the item is being used (post attachment or album). Click on the post title (or album name if it's in an album) and you will be taken to that post/album. As the item will now be in the lower box, you can then use the previous instructions to delete the attachment. If it is located in an album, you can click the "Edit Photo" link to delete it from the album.
As far as "after a certain time the selected will be deleted", I believe you are referring to the phrase "Uploads not utilized within one hour will be deleted" which actually means that if you upload a new item and don't attach it to a post within an hour it will be deleted.
