What does this mean $[X]_1^T$? I found this in information theory paper, P.3883*
the authors states the following

Most existing theoretic studies of network coding focus on DAGs due to its simpler structure and dure to the fact that one can always convert a cyclic network to its acyclic counterpart by taking into account the time index and the causality of information transmission. Nonetheless, when studying Pair Intersession Network Coding on a cyclic network, one cannot rely on this cyclic-to-acyclic conversion since after conversion, there are T symbols in the $(s_1, d_1)$ (resp. $(s_2,d_2)$) multicast session and network coding allows complete freedom of mixing the $2T$ symbols $[X]_1^T$ and $[Y]_1^T$.

It looks ambiguity because does it mean that we have input from 1 to T; or Is it field = T and size = 1 (since capacity = 1)? so I hope I find answer here.
Thank you ..
*for those who doesn't have access go to this site Paper

Responsing to D.W. (The following information is for reference about the paper.)
C-C. Wang and N. B. Shroff. Pairwise Intersession Network Coding on Directed Networks. IEEE Tran. on Info. Theo., Vol. 56, No. 8, August 2010.
This notation is quite common in EE works: 
If $S=S_1S_2S_3\cdots$ is a string (of some length), then

$S_i^j$ is the substring between places $i$ and $j$, that is, $S_i^j=S_i\cdots S_j$.

I didn't looks at the paper, but I guess that $[X]$ is a string (of expectations? of $X$ directly? this should be defined in the paper), and they just look at a prefix of length $T$ of that string.