The textbook which I have wrote the below equation of Z-transform.
$$X(z)=Z[x[n]]$$
From the definition of Z-transform, the rightmost term of sigma is held.
$$X(z)=Z[x[n]]=\sum_{n=-\infty}^{+\infty} x[n ]\cdot z^{-n}$$
Of course the result of the computations of sigma only has $z$ as a variable so I thought the middle term can be rewritten as below.
$$X(z)=Z[x[n]](z)~~\leftarrow~~\text{Added (z) to emphasize that the function has variable z}$$
Is this notation not general or weird or can be subtracted of score of test?
Almost possibly no problem for write $X(z)=Z[x[n]](z)$ in tests however I guess that anybody want to abbreviate $\left(z\right)$ as the frequency of write of $X(z)=Z[x[n]](z)$ increases.