We study different definitions of Sobolev spaces on quasiopen sets in a
complete metric space equipped with a doubling measure supporting a
p-Poincar\'e inequality with 1<p<\infty, and connect them to the Sobolev theory
in R^n. In particular, we show that for quasiopen subsets of R^n the Newtonian
functions, which are naturally defined in any metric space, coincide with the
quasicontinuous representatives of the Sobolev functions studied by
Kilpel\"ainen and Mal\'y in 1992. As a by-product, we establish the
quasi-Lindel\"of principle of the fine topology in metric spaces and study
several variants of local Newtonian and Dirichlet spaces on quasiopen sets.Comment: arXiv admin note: text overlap with arXiv:1410.516
