What niche is PRResearch supposed to fill that the other APS journals don’t already cover?
APS clearly intends for Physical Review Research to be less selective than PRL/PRX, as they elucidated in two volume 1 editorials:
M. Thoennessen: &quot;Editorial: Introducing Physical Review Research&quot;

We are launching Physical Review Research to achieve three main objectives: (1) to cover the entire range of topics in physics and related fields, including interdisciplinary and newly emerging areas of research, (2) to offer an attractive option for all authors who prefer or are required to publish in fully open access journals, and (3) to align its quality standards and selection criteria with those of the other long-established topical journals in the family, such as Physical Review A–E, Physical Review Applied, Physical Review Fluids, and Physical Review Materials.

R. D'Souza, J.-W. Pan, N. Spaldin, and J. J. Liétor-Santos: &quot;Editorial: Our vision for Physical Review Research&quot; in volume 1:

Within the Physical Review family, Physical Review X provides an open access journal showcasing the most selective papers in pure, applied and interdisciplinary physics. Now, Physical Review Research will complement this title by providing a fully open access title with rigorous peer-review standards and with the same acceptance criteria as Physical Review A through E, Physical Review Applied, Physical Review Fluids, and Physical Review Materials. Our goal is to establish a publication that offers the same experience and quality that we value and trust, while increasing authors’ choice within the Physical Review portfolio. As such, we welcome authors of quality manuscripts not accepted by Physical Review X or Physical Review Letters to seamlessly transfer their work to be considered for publication in Physical Review Research.

Hence I would expect its initial impact factor (expected to arrive in June 2021) to be more aligned with the specialized journals, than with the flagships. But as you've already heard it is hard to tell exactly where it will end up.