Query plan on an Index Seek output a IsBaseRow1005
what is that? I googled and didn't find a reference to this.
It is an internal flag, seen in data modification queries and some cursor plans. It helps keep track of what came from where. Update processing is extremely complex. You may also see this flag in plans with CASE and a subquery. Ultimately, it's all internal and nothing to be concerned about.
Came across this page googling "IsBaseRow".
One other place I have seen this used is to avoid performing key lookups on rows that didn't match in an outer join.
A somewhat unfortunate example of that here https://stackoverflow.com/a/46253379/73226