The combination of YEAR, MONTH, SERIAL, and PERNUM uniquely identifies a CPS respondent within a sample. YEAR and MONTH identify the person’s sample, SERIAL identifies the person’s household within the sample, and PERNUM identifies the person’s location within the household.
Unlike with IPUMS-USA, respondents in the CPS appear in multiple samples. Specifically, a household is in the CPS sample for four months, out for eight months, and then back in for four months. The variable CPSIDP can be used to identify respondents across samples. Please refer to this previous answer for more information about using CPSIDP.
Hope this helps.