It's a bit of a kludge, but what they mean is that an area can be analyzed as a unit because an area (unit of land) has a similar geology, climate, elevation, etc. The border of the unit may be irregular, but is determined by the shared geograpical features.
Here is an exerpt from a USFS mapping project that might help with the idomatic usage of "unit of land"
This feature class are polygon layers of map units called terrestrial ecological units (TEU). TEU are mapped units of land within which ecological structure, function, capabilities, responses, and management opportunities and limitations can be predicted (Cleland et al., 1997). TEU are developed by resource specialists through the integration of an ecosystem's elements, i.e.: soils, geology, geomorphology, climate and potential natural vegetation. At LT/LTP scales, these elements are used to describe and define ecological types (ET) that in turn are used to describe and define TEU (see the discussion of this in FSH 2090 for work done to date and, for new mapping projects, the draft TEUI Technical Guide, 1909.xx). TEU are mapped at different analysis scales in accordance with the National Hierarchical Framework of Ecological Units (Cleland et al., 1997), however the LT/LTP scale ecological units are managed on a single GIS coverage. Ecological units (terrestrial and aquatic) are the inventory targets in current Forest Service manual and budget direction.