The purchase pool is the pile of coins you collect in the background. Added to it are coins from trades, income, gift and mining with their respective asset value from that day. Taken out are sales, donations and spends - all of which trigger a capital gains calculation.
The sequence by which coins are taken out of the pool depends on what method choose (FIFO, LIFO etc).
The crucial point is that a deposit does not add to the pool and a withdrawal does not reduce the pool - they are "internal pool movements" with no consequence on gains or taxes.
So if you enter just a deposit for a coin, this coin is a "ghost coin" with no value. It will show up on your dashboard total, but not in the pool for the tax calculations. When it is sold, a warning in the tax report and on the gains page results.
Further information regarding warnings can be found in those articles: