Whenever you see a trade that states "net debit" or "net credit" it will always be in relation to you.  Remember, you are the one placing the order and those are part of your instructions.  For example, if you call your broker and say "Sell the Feb $50 puts and buy the March $50 puts for a net debit of 0.75" that just means that you are willing to pay (debit) up to 75 cents per contract to execute both orders. 

 

In some cases, you may be buying one asset and selling another for a net credit.  This means you are exchanging the two assets plus an additional amount of cash (credit) to your account.  For example, you may have be long stock and short a call (covered call) and wish to "unwind" the position.  You could call your broker and say, "Sell the stock and buy the call for a net credit of x." 

 

It can be confusing at first, but will get easier as you work with it.  The reason traders must specify a net credit or debit is because two assets rarely exchange for exactly the same value.  Because of this, the lower valued side of the trade must be willing to give up some cash in order to make it a fair exchange.  Think about it as if you were trading in a car.  If you are trading in a Yugo and buying a Porsche, it will be for a net debit.  That's because the value of the Yugo is not enough to cover the value of the Porsche so you must cough up some cash in addition to the Yugo.  On the flip side, if you were selling a Porsche and buying a Yugo (not that you ever would!) you would expect a net credit from the dealer.  That means that the dealer must give you the Yugo plus some cash to make it a fair trade.  As long as one side of the trade is willing to exchange cash, any asset can be exchanged for another, which makes the markets much more liquid.  A net debit on one side of the trade must be a net credit for the other trader.

 

Sometimes you may see options whose prices are very close together.  If you wish to do an even exchange, you simply call your broker and say, "Buy this one and sell that one for a net zero (or "net even").  Again, this is fairly rare and most orders require a net debit or credit to be specified.

 

As for the "or better" term.  That's just used in the brokerage business to designate that you are either buying above the ask or selling below the bid.  For example, say an option is bidding $5 and asking $5.50.  You could call your broker and say, "Buy 10 contracts at a limit of $6 or-better."  In this case, the order will most likely be filled for the current $5.50 asking price; however, if the price should suddenly rise, you are allowing 50 cents of flexibility.  Don't worry, market makers cannot arbitrarily fill the order for $6 as they are bound by time and sales, which is an official report of prices at the time your order arrived.

 

In your example, the Feb option was worthless and the March option was priced at 0.70.  The order "Sell Feb and buy March for a net debit of 0.75 or better" just mean that you should execute the trade as long as you don't pay more than 75 cents to roll from Feb to March.