This is a subsidiary post of Editorial Activity of the New Testament.
The New Testament is, of course, a collection of writings. More precisely, however, it is a collection of collections. That is, before the New Testament appeared as a set of writings, those writings appeared in smaller sets (subsets). For example, the four gospels are found together in manuscripts (e.g, manuscript P45). Similarly, Paul’s letters would be combined (e.g. manuscript P46). Scholars sometimes call these smaller sets “collection units,” “circulation units,” “subcorpora,” or similar term.
Kostenberger, Andreas J and L. Scott Kellum and Charles L. Quarles. The Cradle, the Cross, and the Crown: An Introduction to the New Testament. B&H Academic, 2009.
There is an amazing conformity of the manuscripts up until the seventh century in matters of content and order. The NT circulated in four smaller volumes: the four-fold Gospel Codex; Acts and General Epistles; the Letters of Paul (including Hebrews); and Revelation. This order can be seen in the earliest manuscripts (i.e., before the fourth- and fifth -century church councils). [Kindle location 850]
The Fourfold Gospel as a Collection
The Letters of Paul as a Collection
Revelation as a Stand-Alone Writing