[longshadow]

Are they only speaking of the portion of the Universe that we can either see or measure from Earth or by other means?

Yes; "detectable" universe would perhaps be a better terminology. We can only see that portion of the "total universe" that is within our light horizon (the portion of it in which the expansion of space is appears to earth to be at speeds less than the velocity of light.)

[RightWhale]

That's more or less the Hubble volume. The Hubble can't actually see all the way back to the beginning, but 95% or so.