Select this check box if you want the checkouts you make from Developer Studio to be reserved by default. You can also select Unreserved if already reserved to check out unreserved by default if someone else has a reserved checkout for the same element. If you do not select this check box, attempts to reserve an already reserved checkout will fail.
You can change the default for a particular checkout by clicking Advanced in the Check out file(s) dialog box and selecting or clearing the Reserved check box in the resulting Advanced Checkout Options dialog box.