It implements the x86 and x86_64 Linux system call ABI. Linux ELF binaries get vectored to an alternate system call table implemented by the compatibility layer. There are some other components like an implementation of a Linux-compatible procfs. How well it works in practice really depends on how far off the beaten path you go. There are lots of non-essential pieces that are not implemented, but for example I know of people running Steam on FreeBSD.

I've run Oracle JRE and OpenJDK with it and both work OK; also some BlackBerry SDK tools built for Linux. I'm sure there's some rough edges, and I don't know about performance, but once I mounted the appropriate filesystems, things were working, and that was good enough for me. I think that you do have to pick between a current release of FreeBSD and 64-bit Linux binaries or an older release of FreeBSD and 32-bit Linux binaries, and no way to support both sizes on the same host; but that might be me misremembering.

The main thing I want is for OnlyOffice or Collabora to work on FreeBSD in some capacity and I haven't been able to do it (both have open issues that receive very little attention). I want to run my self hosted office solution on the same machine as the data, and I'd really rather avoid VMs.

So, I use Linux because Docker and BTRFS work just fine for me use case. I prefer FreeBSD, but unfortunately I'm unable to solve my problems easily with just FreeBSD, so I'm using something else.