A crossing network will execute client orders at set times throughout the day or when matching buy and sell orders come in. Their order books are not visible so that a large buy order, for example, will not cause sellers to retreat - market impact. Posit uses the mid market prices from the primary orders to determine the matching prices in their system - so that same large buy order, if executed, is guaranteed to get the current market price without slippage.
Communications networks run on a continuous basis - they are fully order driven and their order book is visible to all. They allow participants you see market depth assessing liquidity and potential market impact.