This indicates whether this instrument is primary listed on its stock exchange. There are many dual listed companies on exchanges around the world who have their primary listing elsewhere. The Value of "Is Primary Listing" is set to either 1 or 0 where 1 indicates a primary listing.
This can be a useful flag for identifying companies eligible for inclusion in tax efficient wrappers like ISAs and SIPPs.
To filter for primary listed stocks in the relevant exchange, set this value equal to 1, or to find stocks that are primary listed elsewhere set this equal to zero.