isCartier D
A torus-invariant Weil divisor $D$ on a normal toric variety $X$ is Cartier if it is locally principal, meaning that $X$ has an open cover $\{U_i\}$ such that $D|_{U_i}$ is principal in $U_i$ for every $i$.
On a smooth variety, every Weil divisor is Cartier.
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On a simplicial toric variety, every torus-invariant Weil divisor is $\QQ$-Cartier, which means that every torus-invariant Weil divisor has a positive integer multiple that is Cartier.
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In general, the Cartier divisors are only a subgroup of the Weil divisors.
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