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opam pin is a command in OPAM that allows you to pin a specific version of a package to your local system. This means that whenever you run opam install for that package, it will always install the version that you have pinned, rather than the latest version available. This can be useful if you want to ensure that your project always uses the same version of a dependency, even if a newer version becomes available.
opam install is the main command used to install packages in OPAM. It downloads and installs the latest version of the specified package, along with any dependencies that are required. You can also use opam install to install specific versions of packages, by specifying the version number along with the package name. For example, you could run opam install foo.1.2 to install version 1.2 of the foo package.
In summary, opam pin is used to pin a specific version of a package to your local system, while opam install is used to install the latest version of a package, or a specific version if specified.