This file exists only when BSD-style process accounting is enabled. It contains three values; highwater, lowwater, and frequency. These values control process accounting behavior. If the free space on the file system where the log lives goes below lowwater percentage, accounting suspends. If it goes above highwater percentage, accounting resumes. Frequency determines how often (in seconds) you check the amount of free space.
Default settings are: 4, 2, and 30. That is, suspend accounting if there is less than 2 percent free, resume it if we have a value of 4 or more percent, consider information about the amount of free space valid for 30 seconds.