Assemblage is the process of joining several parcels to form a larger parcel; the resulting increase in value is called plottage.
If we apply these definitions to real estate, which appears to be Mr Reilly's primary domain of expertise, the term parcel refers to parcels of land or other property. He explains why combining parcels increases the total value.
However, Mr Reilly posted these definitions in a blog entitled The Data Advocate, in which he and his colleagues promote the use of data in the real estate business. So we might reasonably use the same terms in the data domain as well. Joining several parcels of data to form a larger parcel (assemblage) is widely recognized as a way of increasing the total value of the data.
While calculation of plottage in the real estate business can be grounded in observations of exchange value or use value, calculation of plottage in the data domain may be rather more difficult. Among other things, we may note that there is much greater diversity in the range of potential uses for a large parcel of data than for a large parcel of land, and that a large parcel of data can often be used for multiple purposes similtaneously.
Nevertheless, even in the absence of accurate monetary estimates of data plottage, the concept of data plottage could be useful for data strategy and management. We should at least be able to argue that some course of action generates greater levels of plottage than some other course of action.
By the way, although the idea that the whole is greater than the sum of its parts is commonly attributed to Aristotle, @sentantiq argues that this attribution is incorrect.
John Reilly, Assemblage vs Plottage (The Data Advocate, 10 July 2014)
Sententiae Antiquae, No, Aristotle Didn’t Write
A Whole is Greater Than the Sum of Its Parts(6 July 2018)