A partial fill on a limit order means only part of your order has been completed - the unfilled remainder stays open and continues trying to fill.
Example: you place a limit order to buy 10 ETH at $2,500 on your Breakout Evaluation account. Only 3 ETH are available at $2,500 or lower, so those 3 ETH fill immediately. The remaining 7 ETH stay pending as an open order.
Your order can continue to fill in stages as more liquidity becomes available at your limit price or better, as long as it remains active. Continuing the example above, the remaining 7 ETH might fill as 2 ETH now and 5 ETH later, completing your original 10 ETH order across multiple fills.
The unfilled portion can also stay open indefinitely if the price moves beyond your limit. If the ask price rises above your buy limit (or the bid price falls below your sell limit), the remainder will not fill until the price returns to a level at or better than your limit price.
Note: Breakout attempts to fill open orders with each new market data update, which occurs roughly every 0.1 seconds. Each attempt depends on the price and size available in the market at that moment, so an update does not guarantee a fill.
The portion of your order that has already filled becomes an open position immediately - it does not wait for the rest of the order to complete. If 3 ETH of a 10 ETH order have filled, your open position is 3 ETH, and any take-profit or stop-loss attached to that position applies only to those 3 ETH, not the remaining 7 ETH.
For more on how your limit price relates to the current market, see: How do bid and ask prices affect my order?