Question about formulas in fluids

[MMCS]

When using bernoullis and momentum, gauge pressure is only used in calculations, any atmospheric pressure is generally omitted, i could understand if, for example, using bernoullis, pressure 1 and pressure 2 were both atmospheric, therefore they would cancel out, but when one pressure is atmospheric and another if different it is still omitted. Why is this?

 

[rude man]

Because atmospheric pressure can still be subtracted out on both sides of the equation.
But you can't have it on one siode and not the other.