"Audit Technique Guides (ATG) (also know as MSSP Guides) . . . provide auditing techniques, assist with issue identification and development, and tax law interpretation. Audit Technique Guides are available to the public and can be accessed at https://www.irs.gov/irm/part4/irm_04-010-007#idm140584513616848." IRM 4.10.7.2.15
"These... [ATGs] help IRS examiners during audits by providing insight into issues and accounting methods unique to specific industries. While ATGs are designed to provide guidance for IRS employees, they’re also useful to small business owners and tax professionals who prepare returns.
ATGs explain industry-specific examination techniques and include common, as well as, unique industry issues, business practices and terminology. Guidance is also provided on the examination of income, interview techniques and evaluation of evidence. So they may be helpful for business and tax planning purposes." ATGs can be found at https://www.irs.gov/businesses/small-businesses-self-employed/audit-techniques-guides-atgs.
Below is a pertinent abbreviation.
Below are links to select sources offering field service advice.