As minoil said don't worry what the broker does, as a trader I designate certain amounts to parcels and keep both a mental note and actual notes on them both on paper and through watchlists i'll add the individual parcels and then sell them off.
So i might buy 5 parcels in a stock. Parcel 1 is for 500,000 shares when I go to sell it i will sell 500,000 shares or if I sell a portion I will make a note and deduct from the parcel the portion I sold off, e.g sold 100,000 so I'll decide to deduct 100,000 shares from parcel 1 leaving me with 400,000 shares. If I buy more in the same stock it becomes a new parcel.
I do T-2 trades so I might buy 500,000 shares and then later that day or the next day sell those 500,000 shares again at a higher price and bank the profits without needing to use any of my own funds, so in that way knowing the parcel is important to me.
This way you know where you're making profits and losses and becomes less confusing tax time. One parcel you might have made $5k profit but another parcel may have a loss of $2k for example so you offset that against the profit made in the other parcel.
Just up to you to designate it how you want. Some holdings I designate for trading short term and others i designate for long term, I could just as easily switch both around, just semantics end of the day.
hope that makes sense and helps answer your question
