As I'm now tracking this order, the credit decreases as the SP moves up and vice versa. I'm trying to wrap my head around this. Explain please?
Closer to ITM = higher chance of ITM = higher cost for the buyer, higher reward for the seller.
You can always decide on a higher or lower credit yourself, but obviously sellers want the highest credit and buyers the lowest, so you average out on a price that's similar for everyone trying to sell puts around the same strike price, taking in account scarcity of the amount of available put contracts, volatility of the SP and nearby of ITM.
Spreads and more complicated strategies are even weirder, because there the credit you decide to sell, is the credit of the sold puts vs the cost of buying puts at a lower strike price (and the credit of sold calls vs cost of buying calls at a higher strike price).
If with puts the SP goes up, the value of those puts lowers because they get further OTM and your unrealised P&L turns green. You can then decide to buy back those puts and cash in the difference. That's what generally is described when closing a position, and with this you can even do day trading with options.
Similar, if you've sold a put spread and the SP goes closer to your strike price, you will see your P&L turn red. If the SP is getting too close to the strike price for your comfort, and you're approaching expiry date, you can then decide to
roll this position over to a safer position. Rolling means you're closing your previous position with the loss, and with a new position possibly recover the loss by choosing a wider spread.
Now, suppose you sold puts which are -20% OTM, and the SP drops to -15%. You will also see your unrealised P&L in the red, but the value of those puts are also dependent on time left to expiration date, the so-called theta. The closer to expiration date, but still OTM, the less value those puts keep, so you might go back to green and keep your credit, as long as your puts never reach strike price by expiration date.