What’s New & Improved Bazzite is now based on Fedora 45! GNOME 51.1 and KDE 6.8.2 Updated OGC kernels to 7.2.15 and 6.18.60 for NVIDIA LTS images. Mesa 26.2.6. Latest and greatest InputPlumber. Optional gaming networking optimization via bpftune and bpfrag services. ROCM OpenCL is now used over Mesa OpenCL on all non-nvidia images. XWaylandVideoBridge has been removed, fixing an entire class of bugs on our KDE images. Acer Predator Atlas 8 initial support: InputPlumber support, Quick Acce...
They refer to the same thing, but people have been preferring “atomic” over “immutable” recently.
Atomic is all-or-nothing upgrades / block upgrades instead of individual packages
Immutable is a read-only operating system root (yes, you have access to /etc, fedora got pedantic about this)
You can have one, the other, or both, or none.
See my reply to OP for sources but there are more than just the two I referenced.