Arc's assignment operations are set up to acquire a global lock as they operate, achieved by use of an (atomic ...) block. This is so other threads can't observe the in-between states of operations like (swap ...) and (rotate ...). The documentation for 'atomic is here: https://arclanguage.github.io/ref/threading.html
Arc also has some support for continuations, which can serve as a kind of cooperative multithreading. Mainly, Arc just exposes 'ccc as a binding for Racket's 'call-with-current-continuation, and it uses Scheme's 'dynamic-wind to implement 'protect. These are documented here: https://arclanguage.github.io/ref/error.html
Those are features in support of concurrency, as in, the interleaving of multiple expression evaluations for the sake of avoiding verbose inversion-of-control coding styles. It looks like racket/place is particularly intended for parallelism, as in, the use of multiple processors at once for the sake of performance. I'd say Arc doesn't provide any particular support for parallelism yet, only concurrency.