What is the universe for the REGIONMEPS variable in IPUMS MEPS?

For the variable REGIONMEPS, I noticed the group NIU’s demographics to be drastically different than the other regions and need to know what may have caused these participants to be categorized at NIU.

The universe for REGIONMEPS is all users who are eligible for data collection during at least one round of data collection during the survey year. The detail can be identified via the ELIG1231(eligibility status as of 12/31 of the survey year) variable. To be in the universe for REGIONMEPS, respondents must have a ELIG1231 value of 4 (Person is eligible). The differing demographics of the NIU group is likely driven by the fact that these individuals are also NIU (and thus ineligible) for some of the other demographic variables.