Position Units are Capitally's implementation detail, not an investing concept. Understanding them is not necessary to use Capitally.
A Position Unit is the smallest segment of an Asset Position. It is created by one opening transaction (such as a Buy, Transfer, or Account Balance) and may be paired with an optional closing transaction (like a Sell, Transfer, Dividend, or Other type).
If a closing transaction only partially covers a Unit, the Unit will be split into two: one that is closed and another that remains open. This pairing process is currently conducted exclusively in a First-in, First-out (FIFO) sequence. This FIFO method enables the calculation of an accurate cost basis for each selling transaction.
Dividends and other income or cost transactions generate separate Units that consist solely of the closing transaction and do not involve a quantity (indicating they are already closed and were never opened).
You buy 10 shares in transaction #1. You have one Unit of 10 shares.
You buy another 10 shares in transaction #2. You now have two Units, both with 10 shares.
You sell 15 shares in transaction #3. The system will:
Close the first Unit using 2/3rds of #3, which is 10 shares, leaving 5 shares to be sold.
Close half of the second Unit with those 5 shares.
Since the Unit can only have one opening and one closing transaction, the second Unit is split in half.
You now have one closed Unit of 10 shares, one close Unit of 5 shares and one opened Unit of 5 shares.