I'm using the following settings on Overleaf:
\documentclass[%
aip,
jmp,%
amsmath,amssymb,
%preprint,%
reprint,%
nofootinbib,
nobibnotes,
%author-year,%
%author-numerical,%
10pt
]{revtex4-2}
\usepackage{graphicx}% Include figure files
\usepackage{dcolumn}% Align table columns on decimal point
\usepackage{bm}% bold math
\usepackage{tabularx}
\usepackage[a4paper, total={6.5in, 9in}]{geometry}
\usepackage{microtype}
\usepackage[english]{babel}
\usepackage[utf8]{inputenx}
\usepackage{bm}
\usepackage{amsbsy}
\usepackage{fixmath}
\usepackage{commath}
\usepackage{float}
\usepackage{siunitx} % notazione \num{e-}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{xcolor}
%\usepackage{subcaption}
\usepackage{hyperref}
\usepackage{curve2e}
%\usepackage{caption}
\usepackage{subcaption}
%\usepackage{silence}
%\WarningFilter{caption}{Unsupported document class}
%\usepackage[center]{subfigure}
\usepackage{comment}
%\usepackage[mathlines]{lineno}% Enable numbering of text and display math
%\linenumbers\relax % Commence numbering lines
and I get the following warning:
Package caption Warning: Unknown document class (or package), standard defaults will be used. See the caption package documentation for explanation.
The thing is that I've commented the caption package (it shouldn't be active anymore), trying to replace it with subcaption.
Is there a way to solve the warning?
subcaption automatically loads the caption package. The warning vanishes if you remove this package as well:
\documentclass[%
aip,
jmp,%
amsmath,amssymb,
%preprint,%
reprint,%
nofootinbib,
nobibnotes,
%author-year,%
%author-numerical,%
10pt
]{revtex4-2}
\usepackage{graphicx}% Include figure files
\usepackage{dcolumn}% Align table columns on decimal point
\usepackage{bm}% bold math
\usepackage{tabularx}
\usepackage[a4paper, total={6.5in, 9in}]{geometry}
\usepackage{microtype}
\usepackage[english]{babel}
\usepackage[utf8]{inputenx}
\usepackage{bm}
\usepackage{amsbsy}
\usepackage{fixmath}
\usepackage{commath}
\usepackage{float}
\usepackage{siunitx} % notazione \num{e-}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{xcolor}
%\usepackage{subcaption}
\usepackage{hyperref}
\usepackage{curve2e}
%\usepackage{caption}
%\usepackage{subcaption}
%\usepackage{silence}
%\WarningFilter{caption}{Unsupported document class}
%\usepackage[center]{subfigure}
\usepackage{comment}
%\usepackage[mathlines]{lineno}% Enable numbering of text and display math
%\linenumbers\relax % Commence numbering lines
\begin{document}
content...
\end{document}