std::memory_order
The C++ argument to an atomic operation that says how much ordering it needs: relaxed, acquire, release, acq_rel or seq_cst.
std::memory_order selects an atomic operation's ordering: relaxed (atomicity only), acquire (loads), release (stores), acq_rel (read-modify-writes), seq_cst (default, single total order); the compiler maps each to plain instructions, acquire/release forms or barriers per ISA (e.g. seq_cst store: x86-64 xchg, ARM64 stlr).
See also