PSU (V021) is the variable indicating the primary sampling unit or PSU. It should be used along with the variable DOMAIN (V023) to account for the impact of the sample design clustering on the variance estimates and standard errors.
The DHS Recode Manuals for Phase 2 forward describe this variable as follows:
Primary sampling unit is a number assigned to sample points to identify the primary sampling units for use in the calculation of sampling errors. This variable is usually the same as the cluster number and/or the ultimate area unit, but may differ if the sample design required a multistage selection process.
See the notes on DHS webpage "Using sample weights" [URL omitted from DDI.] on variance estimation for additional information.