Can someone explain when interactive brokers sell the stock (margin account)?

I'm new to this and I called them several times but they're not giving me a numeric example to understand exactly based on what conditions they sell the stock if the price goes down.
Let's say I have $100k of Apple stock at the price of $100 per share (just example for the sake of simplicity) and I got $200k of margin account (loan against this stock).
Now if the value of my original $100k goes down to X (because Apple share went down to some Y amount), they will sell my Apple stock to collect their loan.
My question is what is that X? Is it a percentage (or any other function) of the original 100k?
Can someone explain this to me?
Thanks