  We show within the framework of relativistic quantum tasks that the doability
of any task is fully determined by a small subset of its parameters that we
call its "coarse causal structure", as well as the distributed computation it
aims to accomplish. We do this by making rigorous the notion of a protocol
using a structure known as a spacetime circuit, which describes how a
computation is preformed across a region of spacetime. Using spacetime circuits
we show that any protocol that can accomplish a given task can, without
changing its doability, undergo significant geometric modifications such as
changing the background spacetime and moving the location of input and output
points, so long as the coarse causal structure of the task is maintained.
Besides giving a powerful tool for determining the doability of a task, our
results strengthen the no-go theorem for position based quantum cryptography to
include arbitrary sending and receiving of signals by verifier agents outside
the authentication region. They also serve as a consistency check for the
holographic principle by showing that discrepancies between bulk and boundary
causal structure can not cause a task to be doable in one but not the other.
