  A significant fraction of all $\gamma$-ray sources detected by the Large Area
Telescope aboard the \fer\ satellite is still lacking a low-energy counterpart.
In addition, there is still a large population of $\gamma$-ray sources with
associated low-energy counterparts that lack firm classifications. In the last
10 years we have undertaken an optical spectroscopic campaign to address the
problem of unassociated/unidentified $\gamma$-ray sources (UGSs), mainly
devoted to observing blazars and blazar candidates because they are the largest
population of $\gamma$-ray sources associated to date. Here we describe the
overall impact of our optical spectroscopic campaign on sources associated in
\fer-LAT catalogs, coupled with objects found in the literature. In the
literature search, we kept track of efforts by different teams that presented
optical spectra of counterparts or potential counterparts of \fer-LAT catalog
sources. Our summary includes an analysis of an additional 30 newly-collected
optical spectra of counterparts or potential counterparts of \fer-LAT sources
of previously unknown nature.New spectra were acquired at the Blanco 4-m and
OAN-SPM 2.1-m telescopes, and those available in the Sloan Digital Sky Survey
(data release 15) archive. All new sources with optical spectra analyzed here
are classified as blazars. Thanks to our campaign, we altogether discovered and
classified 394 targets with an additional 123 objects collected from a
literature search. We began our optical spectroscopic campaign between the
release of the second and third \fer-LAT source catalogs (2FGL and 3FGL,
respectively), and classified about 25\% of the sources with uncertain nature
and discovered a blazar-like potential counterpart for $\sim$10\% of UGSs
listed therein. In the 4FGL catalog, about 350 \fer-LAT sources are classified
to date thanks to our campaign. [incomplete abstract]
