May want to read this link to better understand....
Page 18.
"Manual Ground Balance
After adjusting my Sensitivity setting, I establish the proper GroundBalance (GB). Once again, each model (with the exception of the X-TERRA30, which has a factory preset GB) has a different degree of “fine tuning”for Ground Balance.The 50 and 305 offer Manual Ground Balance with settings ranging from0 to 20. The 505 offers Manual Ground Balance with settings ranging from0 to 50. And the 70 and 705 offer Manual Ground Balance with settingsranging from 0 to 90. The 70 and 705 offer the ability to set your GroundBalance automatically and operate in either a Tracking or Beach Tracking.The 705 also allows the user to offset the Ground Balance setting while inTracking .Ground Balance is the simplest form of discrimination in that it allows youto adjust the ground phase of your detector to compensate for the levelof ground mineralization. In other words, you are setting your detector inan effort to discriminate out the adverse effects of the ground. Note thatthe X-TERRA has the ability to operate effectively in both magnetic andconductive ground.
Ground Balance Settings
X-TERRA 30 Factory Preset
X-TERRA 50 20GB (Manual)
X-TERRA 305 20GB (Manual)
X-TERRA 505 50GB (Manual)
X-TERRA 70 90GB (Auto+Manual)
X-TERRA 705 90GB (Auto+Manual) "
http://www.minelab.com/__files/f/43350/Understanding your X-TERRA Letter size.pdf