From WWDC 2016: Unified Logging and Activity Tracing.
os_log
: Critical details for debugging issues.os_log_info
: Additional information to embed in error or fault reports.os_log_debug
: Optional, high-volume information that can be explicitly turned on during development.os_log_error
: Additional app information is bundled into the log message.os_log_fault
: Additional system information is bundled into the log message.