Why should mass not be represented by an arrow?

I believe that an arrow in physics represents a change from one form to another or or an absorbance of another form of matter or mass. I learned this concept in Middle School on my Phy. Science class. and since mass can change in number it shouldn't be represented as an arrow for that reason, because it can't always have a definite number representation. I believe that's the answer I'm not 100% positive, I hope this helps