The binary packages come from the ports tree and usually the *-devel packages are more up-to-date with dependencies. The easiest way to check what’s the difference between elixir and elixir-devel is to check the port dependencies and/or applied patches. After a quick glance, it looks like elixir depends on Erlang 25 and elixir-devel on Erlang 27.
You can use the tools that FreeBSD provides to inspect information about installed or not installed packages, pkg info and pkg rquery respectively.
