A bit of comparison on the definition of arrangement:
SAA Glossary, as per David B. Gracy, II, Basic Manual Series (1977):
Arrangement is the process and results of organizing archives, records, and manuscripts in accordance with accepted archival principles, particularly provenance, at as many as necessary of the follow levels: repository, record group or comparable control unit, subgroup, series, file unit, and document. The process usually includes packing, labeling, and shelving of archives, records, and manuscripts, and is intended to achieve physical or administrative control and basic identification of the holdings.
Frederic Miller, Archival Fundamental Series I (1990):
Archival arrangement is the process of organizing and managing historical records by (1) identifying or bringing together sets of records derived from a common source which have common characteristics and a common file structure, and (2) identifying relationships among such sets of records and between records and their creators.
Kathleen D. Roe, Archival Fundamental Series II (2005):
Arrangement is the process of organizing materials with respect to their provenance and original order, to protect their context, and to achieve physical and intellectual control over the materials.
Richard Pearce-Moses, A Glossary of Archival & Records Terminology (2005) adds:
(2) The organization and sequence of items within a collection.
He also notes that arrangement is to be distinguished from classification, “which places materials in an order established by someone other than the creator.” He also notes that archivists are not required to preserve “original chaos”.