Explanation
In the main Postfix configuration file, which is usually /etc/postfix/main.cf, service definitions are continued on the next line by starting the following line with white space indentation. This means that the line must begin with one or more spaces or tabs. This is useful when a service definition has many parameters or options that would make the line too long or hard to read. For example, a service definition for the smtpd daemon could look like this:

The first line defines the service name, type, private flag, unpriv flag, chroot flag, wakeup time, maxproc limit, and command. The following lines, indented with white space, define additional options for the smtpd command, such as the syslog name, the SASL authentication, and the recipient restrictions. Each option is prefixed with -o and separated by white space.
References: You can find more information about the main Postfix configuration file and the service definition syntax in the following resources:
Postfix Basic Configuration
Postfix Architecture Overview