For interior plumbing, permits are required only for the installation of new fixtures. The replacement of old fixtures, provided that the replacement fixture goes into the same place without any new plumbing lines, does not require a permit.
For exterior plumbing, a permit is required when installing or replacing a septic system. A permit is not required for minor repairs or for septic tank plumbing. If you are replacing just the tank of your septic system, fill out the "Tank Only" version of the permit.