You have to clarify if you are asking about electronic communication networks or crossing networks. ECN acronym is reserved for the former but CFAI material is also using it for the later in practice problem inadvertently creating this confusion. I think you got responses for two different ECNs.
ECN is a limit-order market with expected anonymity, price discovery and suited for small orders and securities with good liquidity (narrow spreads).
Crossing networks are used by institutional traders executing large block orders. Buy and sell orders are matched at specific times and execution price is midpoint of NBBO (national best bid and offer). Benefits are anonymity, 0 market impact, minimal commissions and wide spreads are mitigated for less liquid securities. Negative factor is that execution depends on availability of large order on the other side of the trade which could delay execution.
Principal trade comes with hefty commissions in form of price concession and fast execution. Suited for wide spreads and large orders.