Short answer

Free cash refers to the balance available in your Arrow account that you can use for placing new trades. The cash margin available also includes any premium you received from short options.

Free Cash = Cash Margin Available + Pay In + Direct Collateral - Margin Used

Components explained:

Cash Margin Available: Funds available in your account for trading

Pay In: Additional funds being added to your account

Direct Collateral: Securities pledged as collateral for margin

Margin Used: Amount currently blocked for existing positions