The XD is correctly classified as a single-action design, meaning it is fully cocked by racking the slide. Most single-action autos have manual safeties, but the XD's Croatian designers eliminated the need for such safeties by adopting the Glock-type trigger safety as well as an internal firing-pin block. They also added a grip safety. The XD is a very safe design, but its single-action designation has confused some.
The XD is a true single-action internally but externally operates more like a double-action-only handgun. There are no manual safeties to disengage beforehand, and the trigger take-up has the feel of a light (very light) double-action pull before reaching a final single-action release. The XD operates like a Glock with a grip safety. As with the Glock, the trigger design does not fit any traditional classification, so Springfield coined its own terminology and dubbed it the USA (Ultra Safety Assurance) trigger safety.
The XD is a single-action design in that the trigger does not cock the striker. But the XD trigger does perform more than a single function, as do many other modern single-action designs. When the trigger is pressed, the trigger bar moves forward, and a lever in the frame pivots upward, disengaging the striker block. It's important to note that this disengagement occurs during the final stage of the trigger release. The striker-safety lever moves up in unison with the sear release. This means the long take-up of the trigger serves no mechanical function. The long pull does enhance safety, however, and performs much the same as a two-stage military-rifle trigger.