Historically BSD offered memory-mapped I/O via mmap() and friends, while System V introduced shared memory segments as part of the new IPC package (shmget() and friends).
Functionally the difference is that memory-mapped I/O is backed by a file while shared segments are not. Shared segments remain allocated after the creating process exits, which is why ...
I think the distinction is between C-standard memory management (malloc, free etc., which descend from Unix v6’s alloc etc.) and memory-mapped I/O, which came to the Unix family through BSD (although it was implemented in SunOS first, apparently, after being documented in 4.2BSD but not implemented there; ultimately the BSD implementation came from Mach). ...
On some Unix-style systems (BSDs and macOS), CtrlT sends SIGINFO to the running process. Some commands handle this directly; otherwise, it’s handled by the kernel, and that’s what produces the output you’re seeing.
SIGINFO on GNU Linux (Arch Linux) missing has more on the topic.
The ACL is needed to set permissions sometimes.
I found a related instruction.
This document on web contents manual exlains about setting ACL for designated user.