Q:

Move tag next to equation in align environment

It's my first time asking here but I have got so much help from this site :D
My particular case is that I need order and tag Maxwell's equations but I need set the tag in a particular location, just after an equation and before explanatory text.
Another question is how could I place (alignment) whole align environment (respect to the page, I mean)?
My code would be this:
\usepackage{amsmath,amsthm,amsfonts,amssymb,graphicx,setspace,multirow,mathtools}
\begin{document}
\begin{align}
&\overrightarrow{\bf{\nabla}}\cdot\overrightarrow{\bf{E}}=\dfrac{\rho}{\varepsilon_0} \nonumber & \\
&\negthickspace
\begin{rcases}
    \overrightarrow{\bf{\nabla}}\cdot\overrightarrow{\bf{B}}=0 \nonumber \\
    \overrightarrow{\bf{\nabla}}\times\overrightarrow{\bf{E}}=\dfrac{\partial\overrightarrow{\bf{B}}}{\partial{t}} \nonumber
\end{rcases} \hspace{0.5cm}\text{Homogeneous equations}& \\
&\overrightarrow{\bf{\nabla}}\times\overrightarrow{\bf{B}}=\mu_0\overrightarrow{\bf{J}} \hspace{0.5cm}\text{Maxwell studies this equation and compares it with}\hspace{0.2cm}\overrightarrow{\bf{\nabla}}\cdot\overrightarrow{\bf{J}}+\dfrac{\partial \rho}{\partial{t}}=0
\end{align}
\end{document}

I want do that because of possibility to do future references to that equation but I wouldn't like that could be confused with the second equation in that line.
Thank you so much! :D

A:

How about this? I put the explanations in a \parbox, and I think there is no risk of confusion.
With the eqparbox package, I align all = signs, and the alignat environment allows alignment of the texts. In addition I replaced \overrightarrow with arrows from the esvect package. Also, I used the esdiff package, from the same author, which simplifies typing partial derivatives (especially higher order derivatives).
\documentclass[a4paper, 11pt]{book}
\usepackage[utf8]{inputenc}

\usepackage{mathtools, amsfonts,amssymb, esvect, esdiff,eqparbox}

\begin{document}

\begin{alignat}{2}
  & \eqmakebox[LS][r]{$ \vv{\boldsymbol{\nabla}}\cdot\vv{\mathbf{E}} $}=\dfrac{\rho}{\varepsilon_0} \nonumber & \\
  &\negthickspace
  \begin{rcases}
  \eqmakebox[LS][r]{$ \vv{\boldsymbol{\nabla}}\cdot\vv{\mathbf{B}} $}=0 \nonumber \\
  \eqmakebox[LS][r]{$ \vv{\boldsymbol{\nabla}}\times\vv{\mathbf{E}} $} =\diffp{\vv{\mathbf{B}}}{t} \nonumber
  \end{rcases} & \hspace{0.5cm} & \text{Homogeneous equations} \\[0.6ex]
    & \eqmakebox[LS][r]{$ \vv{\boldsymbol{\nabla}}\times\vv{\mathbf{B}} $}=\mu_0\vv{\mathbf{J}} & \hspace{0.5cm} & \parbox[t]{6cm}{Maxwell studies this equation \\ and compares it with\hspace{0.2cm}$ \vv{\boldsymbol{\nabla}}\cdot\vv{\mathbf{J}}+\diffp{\rho}{t}=0 $}
\end{alignat}

\end{document} 

