Gradation Air

Gradation Air, or Projection (投影 touei), is the reproduction of an original with prana from the image the user has.

If the image is not perfect, then projection is not possible. Furthermore, the projection is only as strong as the image the user has of it - if the user has a full knowledge of the real one, then the image will be close to reality. But if the image is loose, then its strength will drop and it will vanish.

If you can replicate an original with Gradation Air, then making the replica with the corresponding material is easier and more practical, making the spell useless.

If the real thing was lost and is needed temporarily for a ritual (just a few minutes), projection can be used to reproduce the shape and the substance as closely to the real one as possible.

As the world will try to revise the projection of "something that is already lost", prolonged materialization is basically impossible.

Again, rather than using projection to reproduce everything, it is more efficient to reinforce what already exists as it consumes less prana and is more effective. For example, if a projection requires a prana level of 10, then its strength will only be 3 or 4, will reinforcing something that already exists with the same prana would give something with a strength of 30 or 40.

It is only used to prepare a substitute that will disappear just after when a tool for a ceremony in the present era couldn't be recovered. Projection is nothing more than an empty shadow.

To Emiya Shirou, it is easier to prepare his own projection than pour his prana into an existing object to reinforce it, but that isn't all there is. Emiya Shirou can evaluate the idea behind the creation, determine the foundation, reproduce the base material, copy the manufacture, assimilate the experience, reproduce the age; unlike a normal person who does the recreation from the base idea, he will include in the blueprint the material property, the nature and the history of the item.

With a normal usage of Gradation Air, it is possible to perform a "reinforcement" of an existing object that imitates a "projection", but to Emiya Shirou and the heroic spirit Emiya, it is impossible to compensate with a reinforcement if the image isn't created from A to Z by his Reality Marble.

Source: based on the Fate/Side Material encyclopedia entry (p.67)