How does the Days In Window field work?
As a general rule, you shouldn't have to worry about playing with this, it will be configured to a good default for you.
But this field determines the window of time that a given trigger will look at. Taking abandoned cart as an example, let's say that Days Out is set to 2, and Days In Window is set to 1.
This means that it will be looking at carts that were abandoned between 2 and 3 days ago. If Days In Window was set to 2, it would be looking at carts abandoned between 2 and 4 days ago.
In general, this field will always just be set to 1. But for example if you wanted to send abandoned cart emails out to customers who abandoned in the last 60 days, you could use it to do that.
Another time that this field can come in handy is when you're testing / spot-checking a new trigger. Let's say you create a trigger that's very specific and therefore you don't have any pending recipients that are queued up for it.
You can temporarily increase the Days In Window to a larger number so that it will grab a larger chunk of customers for you to preview and spot-check. Just remember that if you're playing with this and the rule is already published, there's always the chance that the email can be sent out while you're playing with it.