In the old days, “/opt” was used by UNIX vendors like AT&T, Sun, DEC and 3rd-party vendors to hold “Option” packages; i.e. packages that you might have paid extra money for. I don’t recall seeing “/opt” on Berkeley BSD UNIX. They used “/usr/local” for stuff that you installed yourself.
But of course, the true “meaning” of the different directories has always been somewhat vague. That is arguably a good thing, because if these directories had precise (and rigidly enforced) meanings you’d end up with a proliferation of different directory names.
The Filesystem Hierarchy Standard says this about “/opt/*”:
“/opt is reserved for the installation of add-on application software packages.“
By contrast it says this about “/usr/local/*”:
“The /usr/local hierarchy is for use by the system administrator when installing software locally.“
These days, “/usr/local/*” is typically used for installing software that has been built locally, possibly after tweaking configuration options, etcetera.