"3) +1 net change in options for that strike."
So, it means that instead of being short the underlying stock, I am long one call option at the strike price is that correct ?
No. It means you are +1 option at your short strike, net of your original position. If your original position was -1, your current net is [-1 + +1] = 0. If you held a -6 position, it is now -5.
What if the underlying is Futs contract rather than a stock. Such as ES or CL ? Is it the same as stocks? (i.e. I would never be holding (or short) the underlying ? )
It depends on the contract. Look at the specs for each expiry and find whether it is European or American, and whether it finishes in cash or in the underlying. (The ES, BTW, does both.) The math [mkt - strike]*multiplier remains the same. It is good practice to know this stuff before each spread gets written, as in a fast-moving market, you can slip from one expiry to another, not realizing that exercise structure has changed, and "Ooops." is rarely a good thing to hear after a bunch of trades has been done.