Adversaries may abuse Microsoft Office add-ins to obtain persistence on a compromised system. Office add-ins can be used to add functionality to Office programs. (Citation: Microsoft Office Add-ins) There are different types of add-ins that can be used by the various Office products; including Word/Excel add-in Libraries (WLL/XLL), VBA add-ins, Office Component Object Model (COM) add-ins, automation add-ins, VBA Editor (VBE), Visual Studio Tools for Office (VSTO) add-ins, and Outlook add-ins. (Citation: MRWLabs Office Persistence Add-ins)(Citation: FireEye Mail CDS 2018)

Add-ins can be used to obtain persistence because they can be set to execute code when an Office application starts. 
Add-ins
source_name: mitre-attack  url: https://attack.mitre.org/techniques/T1137/006  external_id: T1137.006     source_name: Microsoft Office Add-ins  description: Microsoft. (n.d.). Add or remove add-ins. Retrieved July 3, 2017.  url: https://support.office.com/article/Add-or-remove-add-ins-0af570c4-5cf3-4fa9-9b88-403625a0b460     source_name: MRWLabs Office Persistence Add-ins  description: Knowles, W. (2017, April 21). Add-In Opportunities for Office Persistence. Retrieved July 3, 2017.  url: https://labs.mwrinfosecurity.com/blog/add-in-opportunities-for-office-persistence/     source_name: FireEye Mail CDS 2018  description: Caban, D. and Hirani, M. (2018, October 3). You’ve Got Mail! Enterprise Email Compromise. Retrieved April 22, 2019.  url: https://summit.fireeye.com/content/dam/fireeye-www/summit/cds-2018/presentations/cds18-technical-s03-youve-got-mail.pdf     source_name: GlobalDotName Jun 2019  description: Shukrun, S. (2019, June 2). Office Templates and GlobalDotName - A Stealthy Office Persistence Technique. Retrieved August 26, 2019.  url: https://www.221bluestreet.com/post/office-templates-and-globaldotname-a-stealthy-office-persistence-technique     