When you see a book's status listed as "in transit sublibrary" in the USMAI catalog, that means someone at another UM campus has requested that book and it is currently being shipped to the campus where it was requested. Once it arrives, the person who requested it will be able to check it out.

If you want to request a book that is "on hold" or "in transit" for another borrower, you can still do that.  Just keep in mind that the borrower ahead of you will be able to check out the book and use it until the due date (or recall date).  It may be a few weeks before the book becomes available for you.

To request books within the USMAI system, follow the instructions listed here https://answers.hshsl.umaryland.edu/faq/154357