Here I need to number only one equation in the first align environment in the MWE. I tried it with align*, numbered the equation with \tag. But then cross-referencing was not working properly.
To give side-notes I am using \tag*{}. But the problem appeared when I am adding a label by \label{eq:1} and side-note by \tag*{ [ Using some formulas] }. Because it gives the whole sidenote [ Using some formulas ] when I mention it later as \eqref{eq:1}. But I don't want that. I want to display the \eqref{eq:1} as an equation number.
\documentclass[12pt]{fphw_assignment_toc}
\usepackage[utf8]{inputenc}
\usepackage{lipsum}
\usepackage[sorting=none, defernumbers=true]{biblatex}
\usepackage[
hidelinks, % remove ugly borders around clickable cross-references and hyperlinks
bookmarksopen, % for PDF specific display option
bookmarksnumbered, % for PDF specific display option
]{hyperref}
\usepackage{mathtools}
\usepackage{amsmath, amsthm, amssymb, amsfonts}
%........................
\begin{document}
\begin{align}
P &= ......................................... \nonumber \\
% some lines of equations
&= .........................................\label{eq:1} \tag*{ [ Lorem ipsum dolor sit amet, consectetuer adipiscing elit. ] } \\
&= ......................................... \nonumber \\
% some lines of equations
\implies P &\approx ......................................... \tag*{ [ Using \eqref{eq:2} on \eqref{eq:1} ] } \nonumber
\end{align}
\newpage
\lipsum
\begin{align}
z = ......................................... \label{eq:2}
\end{align}
\end{document}
I want to have an output like:
P = .........................................
= .........................................
[ Lorem ipsum dolor sit amet, consectetuer adipiscing elit. ] (1)
= .........................................
⇒ P ≈ ......................................... [ Using (2) on (1) ]
\newpage ...
\lipsum ...
z = ......................................... (2)
For sidenotes, I have tried using \text{} instead of \tag*{}. But it crosses the right margin when the sidenote is long. I don't want that either. Is it possible to allow auto-break lines in \text{} and align it properly with the available space? Also, I have noticed, if I click on \eqref{eq:2} on pdf, it doesn't take me exactly at \label{eq:2}.
When is the most efficient way to handle these issues?

\documentclass) so we can just copy&paste your code to try some things. – Matthias Arras Oct 06 '20 at 15:07fphw_assignment_toc.clsused in your document? Meanwhile, There is no problem witharticleclass. – Oct 06 '20 at 20:05\textin an equation. The only thing I'd add is that the[t]option to\parboxshould be considered. (Possible duplicate.) – barbara beeton Oct 09 '20 at 13:40