Taken from Investopedia:

Tax lot accounting is a record keeping technique that traces the dates of purchase and sale, cost basis, and transaction size for each security in your portfolio, even if you make more than one trade in the same security (

See How do I handle different tax lot accounting conventions? for how to set up your portfolios to do tax lot accounting in LUSID and Tax-lot accounting methods for more details on the different accounting methods that LUSID supports.