Imagery needs to be corrected for positioning and this depends on whether an accurate digital elevation model is used
To be useful, it is necessary to know where each pixel (image fundamental area unit) is located on the Earth's surface. This sounds like common sense, and while imagery is at the every least provided with some information on its location, it is not always provided with very accurate positioning for each pixel. To get very accurately located imagery, it is necessary to take into account topography to adjust the location of each pixel. This is especially important in areas with significant topography. such as mountain ranges.
The geometrically corrected image (level 2) is registered to a datum and assumes an ellipsoidal surface (i.e. that the Earth approximates a 3D elliptical surface) with no topography. Without taking into account of topography, absolute positioning may be 100m or worse in error, and measuring of distances between points can be very inaccurate as well. Once topography is taken into account, this can be used to adjust the positioning of each pixel to remove these systematic positioning errors but this requires the availability of a digital elevation model (of DEM). The process is illustrated below.
The positioning accuracy then depends on the quality of the DEM. An off-the-shelf "free" DEM might improve horizontal positioning to about 20-30 m. A higher quality off-the-shelf DEM might improve the positioning accuracy to 5-10 m. For the best possible positioning, it may be necessary to create a custom DEM using stereo imagery collected especially for this purpose. This can optimise the positioning to an accuracy of about 1-2 m (or even better with a DEM provided from an airborne survey), although with ground control points (from surveyors "in the field") being required to achieve the best possible accuracy.
Note that if you order imagery with a large viewing angle away from the vertical, then the positioning accuracy and resolution (level of details visible in the imagery) of the imagery will be degraded, particularly in areas with strong topography and/or for viewing angles of over 20 degrees.