It is clearly stated in the documetation for the TransferLog statement:
This directive has exactly the same arguments and effect as the CustomLog directive, with the exception that it does not allow the log format to be specified explicitly or for conditional logging of requests. Instead, the log format is determined by the most recently specified LogFormat directive which does not define a nickname. Common Log Format is used if no other format has been specified.
Go on to the LogFormat statement:
Default: LogFormat "%h %l %u %t \"%r\" %>s %b"
So if a
TransferLog statement is given without any
LogFormat statement the output format is as described above.
If also the
TransferLog statment is missing no access log is written.