As per the MySQL documentation:
The error log contains a record of mysqld startup and shutdown times. It also contains diagnostic messages such as errors, warnings, and notes that occur during server startup and shutdown, and while the server is running.
The error log subsystem consists of components that perform log event filtering and writing, as well as a system variable called log_error_services that configures which components to enable to achieve the desired logging result. The default value of global.log_error_services is log_filter_internal; log_sink_internal:
mysql> SELECT @@global.log_error_services;+----------------------------------------+| @@global.log_error_services |+----------------------------------------+ ...
