FIFO for tax purposes
The rule that a partial sale draws down the earliest purchases first: the cost is taken from the first parcel bought rather than from an average price.
How to read the number
Hence the divergence from a broker's app: the interface shows an average price for the position while the tax is computed parcel by parcel, and the results differ.
When the metric lies
Selling a small part of a long-held position can produce a noticeable tax because the cheapest parcel is the one written off. The order of write-off is set by the tax code and changes with the law.
Also known as: first in first out cost basis