In the of the European INSPIRE directive the following "common principles" are presented:
1. Data should be collected only once and kept where it can be maintained most effectively. 2. It should be possible to combine seamless spatial information from different sources across Europe and share it with many users and applications. 3. It should be possible for information collected at one level/scale to be shared with all levels/scales; detailed for thorough investigations, general for strategic purposes. 4. Geographic information needed for good governance at all levels should be readily and transparently available. 5. Easy to find what geographic information is available, how it can be used to meet a particular need, and under which conditions it can be acquired and used.*
In principle 2 the term "seamless spatial information" is mentioned. Outside references to the above I fail to find good references.
The closest I can get to have this term explained is an old USGS paper discussing "seamless, scaleless cartographic data bases". Their description is as follows:
A seamless data base implies an ability to query, display, retrieve, or otherwise traverse the contents of a large spatial data base without limitations imposed by the spatial extent of the data. For example, a command to display the Mississippi River would yield the entire river, not just a portion of it. A scaleless data base implies an ability to transition from one level of detail to another appropriate to the scale of the display or precision of the data analysis.
I have also found a reference here on gis.stackexchange.com which answers the question on what a "seamless table" is. But that question does not represent a duplicate of this question.
So the question remains - what does seamless spatial information mean in the context of the INSPIRE directive? Or is it simply a consequence of poor use of language where they actually meant to write "[..]possible to seamlessly combine spatial information"?