What does it mean to add `- D` to the command `npm install-D xxx`?

Today, I saw this way of writing
npm install-D sass-loader
. I haven"t found this -D for a long time on the Internet. What does it mean?

Npm
Mar.21,2021

-D is-- the package and version numbers so installed by save-dev are saved in the devDependencies of package.json


npm help install

-D,-- save-dev : Package will appear in your devDependencies.

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