The IRS Audit Techniques Guides (ATGs) focus on developing highly trained examiners for a particular market segment. These Guides contain examination techniques, common and unique industry issues, business practices, industry terminology and other information to assist examiners in performing examinations. 

When I was at the IRS, they were referred to as MSSP (Market Segment Specialization Program) guides.   How are these guides developed?  Teams of IRS Revenue Agents are assigned to audit returns within a particular industry or vocation.  When the audits are over, the IRS Agents get together and write the guides to provide assistance to Revenue Agents who are assigned cases in that particular industry or taxpayer vocation.  

When you are involved in one of these industries or vocations, and are audits (or you are a practitioner representing a taxpayer involved in one of these industries or vocations), it is important to review the applicable guide to learn where the IRS will likely focus its examination.

Here is a link to the IRS website where these guides are maintained:

http://www.irs.gov/businesses/small/article/0,,id=108149,00.html