As far as I know:
- IF you don't put any items on your map through the ruleset, then all UFO Power Sources will automatically generate Elerium.
- IF you do put some items on the map, for example a plasma rifle on a particular tile, then the mechanism won't work and you have to place the Elerium manually.
That mechanism is used in alien bases, where only some UFO Power Sources spawn Elerium because the Elerium is specified to spawn there, and therefore other UFO Power Sources don't have any.