Specifically I'm reading Dirac's General Relativity and he says essentially:

$$ \delta Q = \frac{\partial Q}{\partial x^\mu} \delta x^\mu $$

But what's the difference between this and:

$$ dQ = \frac{\partial Q}{\partial x^\mu} d x^\mu $$

From what I understand this is some kind of variational calculus thing, but I don't explicitly know, it seems completely identical.

Here's the context in which I am reading it from if it helps at all:

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