When you create a new item in your project (e.g. a use case), it gets added to the end of the list in the owning package. You may change the order of items in the project browser by selecting an item (excluding definitions) and clicking the green up and down arrow buttons located immediately above the project browser tree. Alternatively, you can use the keyboard shortcut, Ctrl+Arrow key.
If you would like the ID numbers of the items to match the order shown in the project browser, select Tools / Renumber. On the renumber IDs dialog, select the package(s) you'd like to change the formatting in, set your prefix and starting IDs for each of those packages, then select only those item kinds you'd like to change and click Perform Renumbering.
In the example below, all use cases in the Loan Origination package will be renumbered with a "TLA-" prefix, and will begin at the number 15: