IN PLAIN ENGLISH
For a shared account — you and a partner, a child's savings — so you can look at one person's things at a time without splitting the account in two.
FULL DEFINITION
An optional name you can put on a cash account, a card, a property, a debt or any other thing you've typed in, saying whose it is.
EXAMPLE
Tag the joint ISA cash "Partner" and your own savings "Me": the Portfolio page's Whose menu lists just Partner's rows, and a widget's Filters can leave Partner out.
HOW IT'S CALCULATED
The tag changes nothing about the maths: net worth, every class total and every share still count everything, because it's one account. The Portfolio page's Whose menu only changes which rows are listed. A widget's Owners filter hides every cash account, wallet, card, metal, property, pension, bond, other asset and debt tagged with that name from that widget, exactly as hiding each of them one by one would, so a hidden owner's rows come out of that chart's total too. Shares and funds have no owner: they're held with your trades, not typed in, so they always show. A widget finds a row by the name it displays, so two rows with the same name and different owners can't be told apart — hiding either owner hides both.
Educational content only. It is not personal financial advice.