I saw this formula in following context:
I want to create Banker State. It's supposed to rely on profit from loans instead of taxes. I would like to keep taxes as low as possible.
Reply: what you're trying to do wouldn't work unless you were running massive trade surpluses (a la Saudi Arabia). The government debt represents nongovernment sector equity. (G-T)= -(S-I) -(X-M) The government fiscal deficit represents the nongovernment sector's net financial savings. Government trying to build positive debt means forcing the nongovernment sector to go into debt simply just to pay its tax bill. It doesn't work, it would create vertical unemployment & the automatic fiscal stabilizers would push the government fiscal's position (which is reaction to nongovernment sector spending behavior) into deficit.