Missing data in 9900 tracts?

Hi there, I am using ACS 5-year 2015 data, for basic demographic information (population, race, housing, education). I have about 350 tracts that are missing data, most seem to be in the 9900s (e.g. Census Tract 9900, Baldwin County, Alabama. Census Tract 9900, Contra Costa County, California, etc.). I’m wondering if this was somehow created for boundary changes? Or if there was some other explanation for why these exist at rows, but are empty outside GISJoin related information. Thanks!

There are two factors that might explain the missing data. First, the Census Bureau suppresses data for some records in ACS summary tables, as explained here. Second, NHGIS clips coastal water areas out of its shapefiles. This includes some tracts that are entirely offshore. Tracts that are comprised entirely of water area typically have 99## codes, and vice versa. There may be a few exceptions I’m not aware of, but I think that in all such cases, the population and housing counts are zeros.