Essentially the relation of listing, when one element is added to another forming a series, without any variation in structural relations between them.
It also seems to be known as conjoining. In conjoining propositions they are not chronologically related and are of equal prominence, and there is no need to choose between the two, both are true.
Kovol can form additive relationships by repeating the same final verb with different subjects. This the same pattern as Equivalence, but the subjects are different and not synonyms.
Additive propositions come across as a repeated pattern for Prominence.
Proportionality and Incongruity examples were not found and can be considered additive.
Medium
Addition within sentences is clear to see. How that addition works for proposition clusters and paragraphs is of course, harder to see clearly.
It doesn't seem to differ from Equivalence or Head-amplification in structure, just in content.