First shows up on player's radar like this, titled "Unknown".
When scanned from distance it turns to this icon (Undiscovered Industrial base type of Structure), and title changes to "Structure".
When you walk to the very edge of the base, game pops up a big discovery message telling you full name of the place, eg. "Abandoned relay station". Icon stays dark - Undiscovered.
Only when you actually run around the base, get inside etc. the icon turns to this discovered light version.
Landmarks and Planet Trait POIs have similar multi-stage discovery process.
Most POIs in space turn Discovered when you simply travel to them or to orbit of their planet.
Point Of Interest on a planet, that was not yet identified (by scanning it).
StarmapSurface mapScanner
Landing area Landed ship
Used both for a location on a planet where your ship is currently landed, has previously landed, and a location where other spaceship is currently landed.
Surface mapScanner
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Starport
When next to a city marks that the city has Starport, usually with ship building and Trade terminal.
StarmapSurface mapScanner
Rocky asteroids
Marks asteroid field in space. Asteroids can be mined by shooting them and collecting the resources.