In stock trading, the term partial fill denotes a situation in which a broker order can only be fulfilled in part within a certain time frame. A partial fill may occur, for example, when a broker is only able to purchase part of the desired shares at or below the limit order price. In the case of illiquid markets, a partial fill may occur when there are not enough offers at one time to completely fill large orders immediately.
It is possible that an order may never be completed in full, depending on how the market moves in relation to the order limit or whether new offers become available. In some cases, a full order may only be completed as multiple partial fills over a long period of time.
Adding a fill or kill instruction to orders lets you avoid partial fills, and can be beneficial in some cases. A fill or kill instruction informs the broker that they must cancel the order entirely if they cannot complete it in full.
Each Swiss online broker has its own terms and conditions with regards to partial fills. Some online brokers charge the same brokerage fees for partial fills as for fully-completed orders, which can become expensive when an order can only be completed in multiple partial fills. Other online brokers only charge one brokerage fee for each fully completed order.
Example 1:
You place a buy limit order for 100 shares in a certain stock. The broker is only able to buy 50 shares under the specified limit. The stock’s price remains above the limit so that the remaining 50 shares cannot be purchased.
Example 2:
You place a sell market order for 1000 shares in a certain stock. Your broker is only able to sell 600 shares within that trading day. The remaining 400 shares are sold over the following two days at the highest available bids.
More on this topic:
Swiss online trading platform comparison
Deal of the Day 
