It's a variable used by the ports(7) infrastructure. Per bsd.port.mk(5), which describes all targets and usable variables:
Code:
LOCALBASE
where other ports have already been installed. Default:
/usr/local.
The pkg_add(1) man page is also helpful:
Code:
-L localbase
Install a package under localbase. By default, localbase equals
/usr/local, and specifying it is not necessary. However,
packages can be created using a different localbase (see
pkg_create(1)), and those packages can only be installed by
using the same localbase. See bsd.port.mk(5) for a description
of LOCALBASE.