What does the symbol ∟ mean?

Question

I encountered this symbol $\lefthalfcup$ ∟ in the statement of Besicovitch derivation theorem. It says that the Radon-Nykodym decomposition of the given measure $\nu$ is $\nu=f\mu+\nu^s$, $\nu^s$ is singular with respect to $\mu$ and

$$ \nu^s∟ E $$

where $E$ is a specific set.

I don't understand what that symbol means. It also appears in Federer-Vol'pert theorem and it may have something to do with distributional derivatives as seen here, but I really can't figure it out.

Any help is appreciated, thank you.

Screenshot of page 24 of The Comprehensive $\LaTeX$ symbol list:

Answer 1

Source: Functions of Bounded Variation and Free Discontinuity Problems by Luigi Ambrosio, Nicola Fusco, Diego Pallara.