It works like this:
'Custom POIs' were introduced as a feature on the 27xx series of GPSRs, and are also available on just about every automotive GPSR that has been released since the first 27xx model was released.
A 'Custom POI' is a point of interest (hence POI) that the user has designated themselves. It may be as simple as a latitude longitude, or as complex as a latitude longitude with an associated speed limit and a text message (e.g. 'Camera behind shrub at McDonald's). There is no theoretical limit to the number of Custom POIs that can be loaded into a GPSR - in practice, you can load up to 100,000 or so before you start encountering memory constraints.
Users construct their own Custom POI files in comma separated text format, using other computer programs such as word processors or spreadsheets, then upload these files to the GPSR using a Garmin program entitled 'POI Loader'. Once loaded, the GPSR will provide a visual and auditory warning when the user is approaching a Custom POI. The user can control how far in advance (time or distance) the warning is given.
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The 26xx and earlier series of GPSRs do not support Custom POIs. Some enterprising GPSR enthusiasts came up with the idea of creating their own semi-transparent 'map' files that contained symbols identifying speed cameras. These semi-transparent map files are loaded into the GPSR by the user, and they then present the symbols on top of the conventional map. Garmin had nothing to do with this at all, it was entirely a 'home-brew' user idea.
I don't know how well it works - someone else will have to answer that. I do know that you don't get the visual (i.e. red attention bar) and audio alerts as you get close to one of these symbols - all you see is the symbol on the map.
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If Custom POI detection and warning is important to you, I suggest you get a GPSR that supports it - meaning, a 27xx or newer model.
Michael