When you extract data explicitly from the ribbon and want to extract data from a subset of projects by IDs, you'll specify those IDs for each portfolio group within the Specify IDs for Extraction dialog. This option is handy if you know that only a subset of projects have changed and so you would prefer to extract a subset of projects in the portfolio rather than the whole set using their project IDs to identify them.
Project IDs need to be specified by Portfolio Group. You'll see that each Portfolio Group is listed in the dialog. If you leave ALL as the IDs for a particular group, all projects for that Group will be extracted. If you set the IDs to be blank for a Group, then no projects will be extracted for that Group. Otherwise specify which IDs to extract for each Group in the same way you specify IDs for Portfolio Elements (e.g, "1", "1-3", or "1, 3-4", etc.).