In the context of coding theory, an $[n,k,d]$ linear code is a linear code of length $n$, dimension $k$, and minimum distance $d$. I rarely see it written as "an $(n,k,d)$ linear code". Why? Do the square brackets mean something subtly different?
Something about asking a question always seems to bring up the answer...
The notation $[n,k,d]$ is used for linear codes. An $(n,k,d)$ code may be nonlinear.