This feature is somewhat outdated and not often used.
DATA logging
Data logging consists of periodically logging important data concerning
the operation of the UPS. For the definitive definition of the format,
see log_data() in apcreports.c. The format varies according to the UPS
model and the information available from the UPS.
For UPS models,
NBKPRO, SMART, SHARESMART, and MATRIX, the output is
written in a format very similar to what PowerChute writes. That is:
Any value that is not supported by your UPS such as
AmbientTemperature and Humidity will be blank; the commas before
and after that field will still be output.
An actual example from the log file is:
Nov 2 12:43:05 matou apcupsd[23439]: 224.9,227.5,226.2,27.74,50.00,100.0,30.6,,,226.2