Glossary · Tax
A tax lot is a specific purchase of a security, recorded with its own date and cost, so that gain or loss can be calculated on the exact shares sold.
Lot-level detail is what makes tax-aware trading possible. Selling a position is not one decision but a choice among lots, each with a different holding period and a different embedded gain.
Fragmenting a client's holdings across several registrations fragments the lots too, which is how the loss you needed to harvest ends up in the account nobody was looking at.
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