If you want to strictly follow the FHS, then /var/opt/<appname> for the data of apps installed in /opt is the right place to go. Stuff installed in /usr/local typically just used /var/lib/<appname>.

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The difference between /var/opt and /var/lib (or similarly between /opt and /lib) is usually whether any particular package comes from the standard OS repository or manually downloaded and installed/compiled by the administrator. Again usually what ends up in opt folders is purchased non-free programs and 3rd party software not installed by the OS package manager. Those programs and software usually have their own way of self-management, e.g. satisfying library dependencies, un-installation, etc.