AtoM's default templates are based on the international standards maintained by the International Council on Archives (ICA).
These date type terms are taken directly from the International Council on Archives' ISAD(G) standard. See:
Section 3.1.1 in ISAD(G) outlines how dates should be entered into an archival description:
The SAA Glossary further defines Accumulation as follows:
n. ~ 1. The organic process by which a collection of records grows out of a routine process. - 2. Preservation · A deposit of foreign matter; an accretion.
Accumulation connotes a substance that increased over time, although it may no longer be growing.