Day trading basics

Hi
I am using reliancemoney platform for trading using limitcard Annual 5000.I am holding shares worth about 20 lacs and have been doing devivery trading so far.
I recently started doing day trading(for the first time) and bought 15,000 shares of price 340 under margin trading.
I wanted to buy 10,000 shares more at price 342 but my order was rejected with reason "Margin exceeded: Required X lacs, Available Y lacs for Entity account-xxxx Across Exchange Across SEGMENT Across product"

My question is

1. How is this margin amount(total amount I can buy stocked under margin trading) calculated? Does it depend on this total worth of shares I am holding or total delivery money I am holding or what? and whats the formula to calculate it (expample its equal to 5 time of delivery etc)?
2. Say total margin amount(from point 1) is X and I buy N shares worth whole X amount using margin trading, while selling those N shares later in the day(keeping in mind I have used the whole margin amount mentioned in point 1 to buy shares under margin), would I have problem selling those N shares during day trading?
3) What is the technical term used for this margin amount(point 1)
4) what is margin exposure?

Any pointer for day trading tips would be useful.
thanks
Regards,
New Guy

Margin is borrowing money from your broker for trading purpose.

Exchange provides margin. It varies scrip to scrip. For some you need 12.5% for some you will need 100%. i.e., some scrip you just need 10k to trade worth 100k but for some scrip you need to have whole 100k to trade worth 100k. Check it with your broker, they will provide what's the exposure they are offering to various scrips for day trading.

Related terms are leverage, exposure.

Few brokers offers only cash margin i.e., they will provide margin only for your cash you have with your broker. Some broker along with cash margin they will provide stock margin as well. Here you will get margin for the stocks you are holding in your demat account.