This read-write BITS value is, per its source description, an 8-bit mask deciding whether syslog information is output to the associated device, where each bit corresponds to one severity level and a 1 bit means that level's messages are allowed through. Admins adjust this per device/module combination to fine-tune exactly which severity levels (from the most critical down to debug-level) get logged to a specific output for a specific software module. For example, setting only the bits for the higher-severity levels would suppress routine informational messages from a chatty module while still logging its warnings and errors to that device.