CMMI/SCAMPI/PIID - Practice Implementation Indicator Description

PIID is a mechanism, under CMMI (maturity model)/SCAMPI (appraisal technique), for capturing evidences of a CMMI practice for purpose of appraisal by an external auditor.

PIID are practice evidences that are classified into three types:

• Direct Artifact : a PIID type that is considered a direct output (a direct evidence) of the particular CMMI practice, e.g., the output or product or deliverable of a CMMI process area. 
• Indirect Artifact : a PIID type that is considered an indirect output of the particular CMMI practice, etc. minutes, logs, etc. (the indirect evidences) of the practice, e.g, the CMMI practice log or governance log (I call these secondary output of a process area) 
• Affirmation: a PIID type that includes oral or written evidences confirming the particular CMMI practice (or lack thereof), e.g., some one's affirmation over an interview. 

For instance,

• A "PE Plan" is a direct output of the CMMI "PMC SP 1.5" process therefore it is a Direct Artifact. 
• The associated PE Plan review attendance records are the secondary evidences in the process that produces the PE Plan, and therefore it is considered an Indirect Artifact.
• In addition, if some witness being interviewed over the audit duration asserts that the PE Plan reviewing meeting did in fact took place. It would be considered an "Affirmation" 

To sum it up, if you are an independent audit professional given the responsibility to audit some thing that were supposed to follow CMMI in its entirety, these are the three areas of evidences you will go through to properly execute the audit.   The outcome will be used to prepare the audit report as well as the basis for  formulating your recommendations.