SheetComparePrimary key
A column (or set of columns) whose value uniquely identifies each row. It's what tools use to match records when joining or comparing two datasets.
A primary key uniquely identifies a row: a customer ID, an order number, an email address. By definition it must be unique and present in every row. A key made of several columns together (say date plus account plus amount) is a composite key, used when no single column is unique on its own.
Outside databases, the primary key is the linchpin of comparison. To diff two files, a tool has to know which row in file A corresponds to which row in file B, and the key is that correspondence. Pick a stable, unique key and a comparison can align records even when they've been reordered; pick a non-unique one and rows match ambiguously, producing confusing or wrong diffs. Choosing the right key column is usually the most important setup decision in a reconciliation.