The 2012 ACS releases (2012, 2010-2012, and 2008-2012) use new boundaries for PUMAs. These new definitions are based on new guidelines established by the Census Bureau as well as results from the 2010 Census.
The upshot of the guidelines is that the building blocks for PUMAs must be census tracts or counties. PUMAs can no longer be comprised of places or multiple places, especially as in the case of Michigan these multi-place PUMAs were sometimes comprised of non contiguous places.
The Census Bureau also encourages that the newly constructed PUMAs map to metropolitan areas.
The definition for the composition of PUMAs from the Census Bureau’s site is not all that informative. It is an Excel spreadsheet with the geographic identifier and Name of the PUMA, e.g., Northwest Detroit for PUMA 263208.
To know which census tracts are “Northwest Detroit” one needs to map census tracts to PUMAs. One can do this via the MableGeocorr site [Source: census tract; Target: PUMA2012].
Of course many PUMAs are comprised of multiple counties or a single county, so that sort of detail is not necessary for them. I will update this post later this week with a crosswalk, which includes “census tracts” for multi-PUMA counties and counties for single/combined county PUMAs.