How can i have text between brackets in equation with respect to some criterion?
(edited answer after noticing that I had misunderstood the OP's overall objective)
Something like this?
\documentclass{article}
\usepackage[margin=1in]{geometry}
\usepackage{mathtools,ragged2e,array}
\newcolumntype{C}{>{{}}c<{{}}} % column type for binary and relational operator symbols
\newcommand\mymat[2][0.12]{%
\begin{Bmatrix}
\footnotesize
\begin{tabular}{@{}p{#1\textwidth}@{}} % default width is 0.12\textwidth
\itshape\Centering #2
\end{tabular}
\end{Bmatrix}
}
\begin{document}
\begin{equation}
\setlength\arraycolsep{0pt}
\begin{array}{cCcCcCc}
\partial_t C
& + & \partial_x u C + \partial_x v C
& = & D_x \partial_{xx} C + D_y \partial_{yy} C
& + & R(C) \\[2ex]
\mymat{Time rate of change of concentration of chemical component}
& + &
\mymat[0.1]{Term of advection of chemical component}
& = &
\mymat[0.1]{Change in component due to diffusion}
& + &
\mymat[0.14]{Rate of formation of component~$-$ Rate of consumption of component}
\end{array}
\end{equation}
\end{document}
I guessing that you like text in square brackets related to above equation. For this people usual use something like this:
\documentclass{article}
\usepackage{geometry}
\usepackage{mathtools}
\usepackage{newtxtext,newtxmath} % optional
\usepackage{ragged2e}
\usepackage{array}
\usepackage{stackengine}
\stackMath
\newcommand{\underlabel}[3][9em]{\stackunder{#2}
{\overbrace{\scriptsize
\begin{tabular}{@{}>{\Centering}p{#1}@{}}#3\end{tabular}}_{}}%
}
\newcommand{\mystrut}{\vphantom{\partial_{yy}}}
\begin{document}
\begin{equation}
\underlabel{\partial_t C\mystrut}{Time rate of change of concentration of chemical component}
\ + \ \underlabel{\partial_x u C + \partial_x v C\mystrut}{Term of advection of chemical component}
\ = \ \underlabel[11em]{D_x \partial_{xx} C + D_y \partial_{yy} C}{Change in component due to diffusion}
\ + \ \underlabel[11em]{R(C)\mystrut}{\tabcolsep=3pt
\begin{tabular}{@{}m{5em} c m{5em} @{}}
Rate of formation of component
&$-$& Rate of consumption of component
\end{tabular}}
\end{equation}
\end{document}