When using this target type you will point to a particular page in the current document and you will be able to see the entire page height in the pdf reader of your choice. You can specify an offset from the left margin of a page to point to a particular position in the page, useful especially for large pdf pages in the horizontal direction. If the page is not larger than the viewer width you will not notice the left offset, but it will be there, try to narrow the viewer width and then reselect the bookmark to see the effect. In jpdfbookmarks offsets can be specified in thousandths of the page extension in a particular direction, so if you want to point to the middle of the page you have to specify a left offset of 500. To be more precise i recommend the use of the left ruler, for instructions on how to use it read the relative section. Another option is to inherit the left offset from the previous bookmark, i don't find it very useful but try to experiment with it if you want. Obviously with this kind of bookmark the top offset setting is not available and is disabled in the dialog.