Fill
A fill is an executed portion of an order. One order can have one fill or many fills at different prices and sizes.
Definition
A fill is an executed portion of your order. A single order may be filled all at once or split into multiple fills depending on market depth and matching.
Why it matters
Multiple fills at different prices change your true execution price. For cost measurement, combine fills into a single execution price using VWAP.
Common pitfalls
Assuming one price: Traders often remember the top of book price they saw, not the actual fill breakdown.
Not recording quantities: Without quantities you cannot compute VWAP correctly.