The sword (Ex) – the ultimate creation of Emiya Shirou, this is an artifact which existence transcends that of any other inanimate object. Made using the notes that his adoptive father Emiya Kiritsugu left behind on the creation of his own conceptual weapon - origin bullets, it is by far superior to any other artifact crafted by modern magi and is, in fact, infinitely close to the level of mystery that is true magic, despite being crafted by the man that, at the time, could be rightfully considered a total failure in the arcane arts.
Using Shirou's rib as a base material, The Sword was forged using the everlasting flames of the infinite manufactory that is his soul, called Unlimited Blade Works. There, the part of his body holding the origin of "the sword" and flooded to the brim with the reinforcing magic bearing the elemental nature of "the sword", was cleansed of all imperfections and unnecessary additions polluting the cleanness of the conceptual existence that became and forever was The Sword.
The Sword is the distilled and concentrated concept realized through the application of forbidden magic that is a reality marble and anchored to the world using the body of it's creator. Because it is the pure idea of "the sword" materialized into existence that has no other additions, it possesses several properties that are miraculous and unobtainable by any science or magecraft accessible to the modern day magi. By itself and without any external forces, it is a sword that does not know the concept of breaking, shattering or dulling. It is a supreme blade able to cut through anything that can potentially be cut. This is, however, just the most basic and unimpressive of it's properties.
Since all objects that has ever bore and that will ever bear the name of "sword" inevitably contain The Sword as the basis of their existence and all of them are encompassed by the concept of "sword", there exists a connection between them and The Sword. In some way it can potentially be said that The Sword is, in fact, all swords to have ever existed and all the swords that could ever exist. This, in turn, means that The Sword contains all of their properties and history and that the bearer of The Sword could potentially access all those powers, abilities and that knowledge, if he or she studied The Sword intently enough. In most basic form this allows the replication of properties of any blade ever seen by the wielder of The Sword by the transformation of the Sword into the perfect replica of the blade in question.
The other miracle of The sword is by far more personal. Since it's creator Emiya Shirou is himself, a Sword in both element and origin, The Sword's existence encompasses his life. Thus, as long as The Sword exists, the being known as Shirou Emiya will exist in this world. This grants Shirou existence unlike any other, and, coupled with the creation of the artifact that cannot be considered anything else but a true miracle with the connection to the legends of all swords in existence, elevates him to the level where he can be called a Heroic Spirit.
If heroic Spirit Emiya Shirou was to be summoned into the Grail War, he would inevitably be summoned as a Saber class servant. In fact, it could be questioned, who exactly would be summoned – a human by the name of Shirou famous for the creation and use of The Sword, or The Sword itself, containing within itself the human Emiya Shirou.