On a chemical reaction, we always have formulas on the left and right parts of the equation.
On the left, we have the reactants and on the right we have the products.
An acid is simply a type of chemical, so an acid can appear anywhere on a chemical reaction.
A catalyst is a chemical that participates on the reaction but it is not consumed. The reactants for instance, participate and are consumed.
Because catalyst are not consumed like reactants, thay need to go on another part of the equation so we don't confuse it with the other. Its place is on the arrow of the reaction.
Thus, the fomuala on the arrow is a catalyst.