Lots are composed of all the Position Units of an Asset that were were initiated by the same Transaction.
Every time you execute a purchase, you open a new Lot. Should you sell only a portion of this Lot, the sold part is deemed closed, while the remainder continues to be open.
A Convert / Move / Spinoff that carries its lots over is the one transaction that opens several Lots at once. Moving a holding to another account, or converting it into another asset, opens one Lot in the destination per Lot that left, each keeping the acquisition date, cost basis, acquisition fee and paid tax of the purchase behind it. Two Lots arriving in one move stay two Lots, so a later sale is still matched purchase by purchase.
For a more detailed explanation, you can refer to the Investopedia article on Tax Lots.