If you haven't enabled unified buying power, the available amount for ETF subscriptions will be the sum of your current account cash withdrawals, outstanding amounts, and collateral for your positions. It will also be subject to the margin's maximum limits, etc.
If you have enabled unified buying power, the available amount will also include the buying power generated by the collateral of assets in your US/A-share account.
The assets in your US/A-share (including cash and stocks) will be used as collateral to generate the buying power for the HK stock ETF subscription. If you have $100,000 cash in your US margin account, based on the HKD/USD exchange rate of 1: 0.13, it will generate approximately HK$770,000 subscription buying power.
If you have stocks with a market value of USD 100,000 in your US stock margin account, the position margin will be deducted from the market value, and the remaining part generates HKD buying power. If the initial margin rate for the stock is 50% and the HKD/USD exchange rate is 1: 0.13, it will generate HK$385,000 of subscription buying power.
(The above content is only an example. The actual buying power will be subject to risk control rules, and the actual amount available shall be subject to the display of the subscription page.)
The final HK ETF IOP buying power generated by the collateral of assets in your US stock account will be subject to your cash holdings and long and short positions.