TMdict

The Miniature World Enclosed by the Azure Sky. Achilles’ trump card which protects the wielder by deploying a miniature world. A divine armament created by the hands of the blacksmith god Hephaestus. It can event against nearly any attack, whether it be from an Anti-Unit, Anti-Army, Anti-Fortress or even Anti-Country Noble Phantasm. However, due to this Noble Phantasm’s nature, it is weak against Anti-World Noble Phantasms alone.
A description of this shield appears in a hundred lines of text in Book 18 of the “Illiad.” Hephaestus reproduced the world he lived in itself in miniature form in this shield.
In the light novel, Achilles doesn’t use it himself, gave it over to Astolfo for him to use. Naturally, transferring Noble Phantasms would normally never happen in a regular Holy Grail War. Even in the Great Holy Grail War, it would normally be unthinkable.
In the first place, most Noble Phantasms are tied to the legends of heroes. Even if you were to borrow Gae Bolg from the blue spearman, that doesn’t mean you would be able to activate Gae Bolg.
However, there are exceptions. In this case, in addition to the necessary conditions of “being where he is making a contract, instead of being in opposition, of his own volition” and the Noble Phantasm “not requiring considerable skill and power to invoke its true name,” it is believed that the transfer of the Noble Phantasm went smoothly because there is “an episode in the legend of the giver (Achilles) where he loans his Noble Phantasms” and “an episode in the legend of the receiver (Astolfo) where he borrows a Noble Phantasm.”
In addition, when Achilles alone wields it, this shield can be used to “attack” as well. After deploying the Noble Phantasm, he would aim to crush the enemy with that miniature world by continuously pushing forward. Most likely, Hephaestus never thought of such a method of using it.