What is the roles and responsibilities of prinicipal switch?
the principal switch manages the assignment of uniqe domain IDs and provides time synchronization to all other switches in the fabric.
The time for management purposes is not done over the fabric you need NTP for that. Cisco can do this via CFS but Brocade has no such feature. In addition to DID assignment. it is also responsible for all fabric related changes, zone locking, database distributions etc. By default the switch with the lowest WWN will become the principle switch but you can configure the preferences manually. Typically you want to have the switch of the latest generation in the fabric to become the principle. Check the "fabricprinciple" CLI command on how to configure this.