The fabricated spiritron world that was made inside Moon Cell.
It’s a “moon-powering, city-like engine” made to manage the inside of the massive Moon Cell.
In short, it is an additional engine and additional memory that Moon Cell added on while expanding itself.
From the perspective of a normal human, Moon Cell’s network is only seen as light, but a magus who can transform their soul into spiritron is able to perceive the information held within that light, and join in as a unit attached to the network. SE.RA.PH is also used to carry out the Holy Grail War, which is for the purpose of better knowing and understanding humans.
SE.RA.PH is scattered around within Moon Cell, and includes, among other things, parts which are already old, obsolete and abandoned, and also independent areas which still haven’t collapsed.

Fate/EXTRA Material: Encyclopedia of Fate/EXTRA

The virtual spiritron world that was created within the Moon Cell.
When the Moon Cell grew to a great size, it created this city-shaped engine in the form of an additional layer in order to manage itself.
A human can only view the Moon Cell’s network as pure light, but a Wizard who can turn his or her being into spiritrons can perceive a network of pure information, and join as a node of the network.
The “Moon’s Holy Grail War,” which was held for the purpose of understanding humans, made use of SE.RA.PH as well.
SE.RA.PH was fragmented within the Moon Cell, and although some pieces were deemed obsolete and deleted, there were still individual areas that have yet to crumble. However, when it was reborn as the new SE.RA.PH, all of the scattered territories merged together, and they are still in the process of becoming one giant open world.

Fate/EXTELLA material: Encyclopedia of Fate EXTELLA