[Spivak]

This isn’t technically correct when Flatpak is installed via your distribution. Then it is managed by your packaging system and it’s data should live in a /var/lib.

/opt is for software that doesn’t adhere to the FHS and can’t/shouldn’t be installed in a prefix. Red Hat and Arch install software to /opt via their official package managers. /opt isn’t safe to stick arbitrary software even though it’s commonly used for that — /opt/local is the non-FHS equivalent of /usr/local.