FIFO
Regulation & taxAlso called: First in first out, First-in-first-out, FIFO matching
First in, first out — the accepted basis for identifying which shares or units were sold when a holding was built up in tranches.
In plain terms
You do not get to nominate the expensive lot. Sell part of a position and the earliest purchases are treated as the ones sold, which sets both the cost and the holding period applied.