From the Delphi code

To output information to the QUARK.LOG, use the Log-call defined in the Logging.pas file. There are several ways it can be called. The first parameter is optional and indicates the part of the code trying to log an item. It is of the TLogName type:

TLogName: LOG_DEFAULT, LOG_PASCAL, LOG_PYTHON, LOG_SYS, LOG_DEBUG.

The second, also optional parameter indicated the severity of the item to be logged. All calls to Log with a level equal or lower to the level- setting will be logged, the rest will be ignored. The default logging level is 20. To set the log level, see the information about Environmental variables below.
The rest of the parameters is the information to be logged. This can be either a single string, or a string with format information followed by all the variables to put in. See information about the Format function in Delphi.

Internally, QuArK uses aLog to actually output the information to the log file. This procedure should not be called directly.

Environmental variables for Logging

QuArK reads several environmental variables to control logging. These are not set via a configuration file, because a problem with the file could cause a failure to create a logfile. Environmental variables can be set in the operating system. The following variables are related to logging and can be set:

QUARK_LOG_FILENAME: The name of the file to log to. This file will be cleared when QuArK starts up.
QUARK_LOG_PATCHNAME: The name of the log of a patcher. Shouldn't be used anymore.
QUARK_LOG_LEVEL: The log level of QuArK. See above in the 'Delphi code' part for a description.

See 'Logging.pas' for more on logging.