Final answer:
User specified R/RSI values are used in building assemblies where standard values do not apply, with innovative materials, or when precise energy modeling is required, typically requiring professional analysis or testing.
Step-by-step explanation:
The circumstances when 'User specified' R/RSI values can be used for building assemblies typically involve situations where standard R/RSI values do not apply or where specific, detailed knowledge of the materials and their insulation properties are known. This can happen when innovative materials or advanced assembly methods are used that have not been encapsulated in the current building codes or when highly accurate energy modeling is required for a building project. In such cases, an engineer or a qualified professional might perform detailed calculations or testing to determine the specific thermal resistance (R-value) or thermal resistivity (RSI value) for those materials or assemblies.