Constant SYSTEM_DEFAULT 

pub const SYSTEM_DEFAULT: QoSProfile;

rmw_qos_profile_system_default — an absence, not a profile: every policy is the SYSTEM_DEFAULT sentinel and the depth is the depth sentinel, 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 was the first of these to become an alias, and since W10 it is no longer the only one: see QoSProfile::QOS_PROFILE_SYSTEM_DEFAULT.