Its not Kb or Mb, its just the number of Bytes.
A traffic log records the following items for each session:
- Date and time of the message
- Message type (session-init or session-close)
- Source address and port number
- Destination address and port number
- IP information
- Session index (sid)
- Policy index (pid)
- Bytes sent and received
- Session duration
You can find details on http://kb.juniper.net/InfoCenter/index?page=content&id=KB16509&smlogin=true&actp=search