When you place an order, it needs to be matched with another order (or multiple orders) to complete the trade.
For example, if you want to buy 100.56 Apple shares, your order is matched with any open orders to sell Apple shares. The 0.56 fractional part of your order will always be sold to you from Drive Wealths account. The full share part can be filled in 2 ways:
Your buy order is matched with one sell order on the share markets for 100 Apple shares.
Your buy order is matched with multiple sell orders on the share markets, over the course of a market day. Each sell order would partially complete your buy order (i.e. a sell order for 50 Apple shares, another sell order for 20 shares, and a final sell order for 30 shares). In this case, you receive an email every time your order is partially filled and a final email when your order is completed.