Homebrew can allow multiple versions of a formula to be installed. For example, there is formulae called node and node@10 and similar.
$ brew info node@10
...
==> Caveats
node@10 is keg-only, which means it was not symlinked into /usr/local,
because this is an alternate version of another formula.
If I have both node and node@10 installed, where node is at v11. I can decide later to active the earlier version with brew link:
$ brew unlink node
$ brew link node@10
$ cd /urs/local/bin
$ ls -l node
lrwxr-xr-x 1 user admin 34 12 Dec 20:07 node -> ../Cellar/node@10/10.14.1/bin/node
Here the symlink node is pointing to an earlier version (keg-only) installed in Cellar.