ARC is global, shared by all ZFS pools on your system. You’ll often want to set a maximum size just to avoid freaking out monitoring systems.
ZFS ARC doesn’t show as cached memory but instead as used, even though, just like cache, it will get freeed as memory pressure increases on the system. The result is that monitoring systems will think you’re running out of memory when it’s in fact just ZFS doing a bunch of caching and applications still very much being able to allocate memory if they need it.