What is "main" parameter for nginx access_log?

In many nginx config tutorials I see this:

access_log /var/log/nginx/access.log main

What is main? It is not found in the docs.

The second parameter of the access_log directive is an optional format name, which determines the content of a log file entry.

The default format is combined, which is predefined as:

log_format combined '$remote_addr - $remote_user [$time_local] '
                    '"$request" $status $body_bytes_sent '
                    '"$http_referer" "$http_user_agent"';

See this document for details.

The format named main should be defined by a log_format directive elsewhere in your Nginx configuration. Use the command nginx -T (with an uppercase T) to view the entire Nginx configuration across all included files.