The following holds true unless the woman has a mutation of her organs:
If it comes from the Urethra, it's coming from the urinary bladder. No anatomical way around that.
If it's less than a teaspoon of lubricant-like fluid, it comes from the skene's glands.
If it comes from the vagina, it's whatever you put in the vagina in order to squirt out.
I'm not saying there's not exceptions - there always are - but the vast majority of female projectile squirting is piss.
Anyone whose bothered to study human anatomy for a few minutes and most adult-content directors could tell you that.
