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 chosen (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.
The purchase pool are based on transactions. Buys or Income fill and sells or expenses reduce the pool of coins - and if you use depot separation (Depot/Lot separation) those pools are logically build for each exchange. Without depot separation filter there would only be one pool per coin for your whole account.
So if you buy 0,1 BTC and later again 0,1 BTC which leads to a pool ("pile") of 0,2 BTC and you sell then later 0,05 BTC you have a pool left of 0,15 BTC. And if you would miss adding both buys before and add only the sell you would receive a purchase pool warning across the reports. To check what is wrong please use the reports described here: How to validate my account?
Further information regarding warnings can be found in those articles: