Yes equity is as I said before a function of your balance and your P/L as is your useable margin. Your usable margin is your balance plus your P/L minus initial margin, or your equity minus initial margin. I.e. Usable Margin = balance + P/L - ...

Your balance is what you have in your account after your last closed trade. Your equity is a function of your balance and the P/L of your open trade(s). Unless you have a credit arrangement with your broker you cannot have a negative balance since ...