The optional dependency specifies that if the given package is installed, it must be at the given version or greater.
This type of dependency is typically used to handle cases where packages transfer content. In this case, each version of the package post-transfer would contain an optional dependency on the post-transfer version of the other package, so that it would be impossible to install incompatible versions of the two packages. Omitting the version on an optional dependency makes the dependency meaningless, but is permitted.
An example optional dependency is:
depend fmri=pkg:/x11/server/xorg@1.9.99 type=optional