The inline comments in the lightstreamer_log_conf.xml file contain all the relevant information.
The syntax is the one managed by Log4j; for severe errors, the priority name used is "FATAL", though they are only logged upon Server closure.
At "ERROR" priority, error conditions that may affect single sessions or single client requests can be logged.

You probably refer to
If possible, a smaller number of items should be used when taking the log.
Actually, I just proposed using a different page or manually changing the subscription.
There is no facility for selectively logging only a subset of the items or sessions.