Which statement does not describe benzene, $\ce{C6H6}$?

$ \textbf{(A)}\ \text{It is an aromatic hydrocarbon.} \qquad$
$\textbf{(B)}\ \text{It exists in two isomeric forms.} \qquad$
$\textbf{(C)}\ \text{It undergoes substitution reactions.} \qquad$
$\textbf{(D)}\ \text{It can react to form three different products with the formula C}_6\text{H}_4\text{Cl}_2\qquad$
1. **Option (A):** Benzene is indeed an aromatic hydrocarbon. Aromatic hydrocarbons are compounds that contain a planar ring of atoms that are stabilized by a delocalized π-electron cloud. Benzene, $\ce{C6H6}$, fits this description perfectly as it has a hexagonal ring structure with alternating double bonds that are delocalized. Therefore, statement (A) is true and does describe benzene.

2. **Option (B):** The statement claims that benzene exists in two isomeric forms. This is incorrect. Benzene has a single unique structure with a hexagonal ring and delocalized electrons. While benzene can be represented by two resonance structures, these are not isomers but rather different ways of depicting the same molecule. Therefore, statement (B) is false and does not describe benzene.

3. **Option (C):** Benzene undergoes substitution reactions rather than addition reactions. This is a characteristic feature of aromatic compounds. For example, benzene can undergo electrophilic aromatic substitution reactions such as nitration, sulfonation, and halogenation. Therefore, statement (C) is true and does describe benzene.

4. **Option (D):** Benzene can react to form three different products with the formula $\ce{C6H4Cl2}$. These products are ortho-dichlorobenzene, meta-dichlorobenzene, and para-dichlorobenzene. This is due to the different possible positions of the chlorine atoms on the benzene ring. Therefore, statement (D) is true and does describe benzene.

Conclusion:
The statement that does not describe benzene is (B).

The final answer is $\boxed{\textbf{(B)}}$