When executing additional positions with the same instrument, they are added or reduced to/from the initial position opened regardless of their direction - Buy or Sell. This is called position Netting.
For example, if a Sell position of 0.5 BTC with BTC/USD instrument is opened, and an additional Sell position of 0.5 BTC is executed, it will be added to the initial position resulting in a total of 1 BTC Sell position.
And vice-versa. if a Buy position of 0.5 BTC with BTC/USD instrument is opened, and an additional Sell position of 0.2 is executed, it will reduce the initial position resulting in a total of 0.3 BTC Buy position.