Available cash: It consists of funds added from your bank account, funds received from selling portfolio holdings, profits/loss earned and opening cash balance.
Available cash shows your real time cash balance and keeps changing throughout the day if you've made any trades.
Your cash balance can become negative if you have open MTF positions, unpaid stocks or open Intraday/ FnO position.
When you buy anything on delivery you're assigned a margin value ( a certain %age of value of holdings) for having those shares, by default. By pledging this margin amount increases. This margin can be utilized to do intraday.
Margin from pledged holdings: It is the additional balance you receive when you pledge your stocks.
This margin is available to your for trading after deducting a HAIRCUT (margin of safety) from your pledged stock value.
This balance/margin can be used to trade in intraday, take short options/futures positions and MTF.
This balance cannot be used to buy stocks in delivery (long-term). For that, please refer to "Available Cash"