As far as I know, Guardian is only one of the overlays available. It is very popular but it has more friction than a wood lane.
It's called "overlay" because it's a thin plastic sheet that "lays over" the existing wood surface that, hopefully, has been sanded smooth and filled in properly, so it is very smooth. It's done because the wood surface is very old and cannot be taken down any longer or is in bad shape and the owners choose not to put in full synthetics. Sometimes it's put over the entire 60 feet of lane. Sometimes it's put on over only the heads and maybe 30 feet down.
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"None are so blind as those who will not see."