GISJOIN field for which census year

I downloaded data from NHGIS for 2005-2009 ACS 5-year estimate at the census tract level.
I am wondering whether the GISJOIN field contains the 2000 or 2010 census tract boundaries?

In most counties, the tracts in the 2005-2009 ACS data correspond to the 2000 definitions, but there are several exceptions. See the note here about our 2009 tract and block group boundary files for related information. See also the Census Bureau’s listing of significant county boundary changes in the 2000s, which may also impact the codes used to identify census tracts in the 2009 ACS.

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So, just for my own understanding, if I download data for 2015-19 ACS 5 year estimate, the GISJOIN file will correspond to the 2010 census tract boundaries.
Please let me know if this is correct.

Yes, that’s mostly correct… All ACS 5-year releases from 2006-2010 through 2015-2019 use 2010 census tract definitions, except that the GISJOIN for tracts includes a county code, and there are a few changes in county codes between censuses. The Census Bureau lists all significant changes to counties on the page I linked to earlier.

Thank you so much for the information @JonathanSchroeder
Really appreciate your efforts.