Existing posts regarding tagging for aligned I've found are all about numbering/referencing, but that's not my concern here.

I often would like to add remarks or short phrases at the end or beginning of equations in the aligned environment (inside a bigger math environment), like saying "substitute $y = f(x)$", "or", "denote $u \equiv 1/x$", etc.

Individual lines in aligned doesn't allow \tag. Most of the time I just use \qquad \text{my remarks} at the end. Sometimes I can get by with the optional placement [t],[c],[b] to fit the \tag for the whole aligned to the desired line.

What would be a good practice to do this? Or is it just a habit I should get rid of, commenting along with the equations when using aligned?

Thank you.

P.S. MWE is not needed for this...I think?

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    You always provide a mwe as common courtesy. Without seeing an example, I would probably never include that sort of remarks inside the displayed math. Always outside for clarity. But again there can be exceptions – daleif Dec 25 '16 at 8:32
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    Yes, a MWE is needed. At least an approximation of what you'd like to get. – egreg Dec 25 '16 at 9:49
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    See tex.stackexchange.com/q/27283/15925 - particularly the solution with the most votes using align, rather than the accepted solution. – Andrew Swann Dec 31 '16 at 13:19

If the explanations can be written in the body text, that is usually to be preferred. In certain cases, however, small remarks can be beneficial. I have used it when teaching linear equations as shown in the image below - written in Danish. Describing the steps of solving equations

If this is the effect you want, add another alignment character in your align-environment.

    3(2+x)  &=x+8       &\\
    6+3x    &=x+8       &\text{(Gang parentesen ud)}\\
    6+3x-x  &=8         &\text{(Saml $x$'erne på samme side)}\\
    2x      &=8-6       &\text{(Isolér $x$'erne)}\\
    2x      &=-2        &\\
    x       &=\frac{-2}{2}  &\text{(Divider med koefficienten foran $x$)}\\
    x       &=-1.
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