Why does CALCULATETABLE not register as a single column when used as TREATAS

When doing it without just CALCULATETABLE like this:

VAR RewnewedPolicies =
    CALCULATETABLE(VALUES(Fact_VT[PN]), ...)

you will get a single-column table, but the column retains its original metadata and name (Fact_VT[PN]).  TREATAS cannot reliably bind a source column with metadata tied to another table into a new context without ambiguity.

SELECTCOLUMNS flattens this ambiguity, you get a column with a defined name ("__PN")

So, effectively, SELECTCOLUMNS is your insurance policy that the shape of the table matches TREATAS' expectations.

Please mark this post as a solution if it helps you. Appreciate Kudos.