pub const SYSTEM_DEFAULT: QoSProfile;Expand description
rmw_qos_profile_system_default-equivalent — an absence, not a
profile: every policy is the SYSTEM_DEFAULT sentinel and the depth is
0, resolved by whichever backend is linked.
issue 0829 — this said = DEFAULT (depth 10) while
QoSProfile::QOS_PROFILE_SYSTEM_DEFAULT said depth 1, so one name
shipped two queue depths depending on which spelling a caller reached.
It is now an ALIAS of the associated const, like the other four, and
neither number survives: see QoSProfile::QOS_PROFILE_SYSTEM_DEFAULT.