Heirloom gets used loosely, but strictly it means a line that left its region of origin and has been maintained by growers elsewhere, whereas a landrace is still adapted to and grown in its home valley. An Afghan indica kept in California since the seventies is an heirloom, not a landrace, however pure it is. The distinction, and why it matters when a seed vendor charges landrace prices, is explained on https://kushindicaseeds.com with examples of each. Either can be great, but the label should be accurate.