@Didi

Welcome flow is usually for new subscribers, and usually for people who never purchased. Everyone has the post purchase flow so that is why they usually don’t want people who placed order in the welcome flow. That is the basic practice but doesn’t have to be like that and you should definitely change things according to your needs.

Now about what each of those 2 filters do:
 - Placed order zero times over all time - what this does is this will exclude people who ever placed order from your Welcome flow and this is the basic set up as I explained above because it is expected those people will go through the post purchase flow and we don’t want to overwhelm them with to many emails

 - Placed order zero times since starting this flow - this is similar filter with one difference it will exclude people who placed order after they started this flow and would not care if people placed order before starting it, where in the first filter above it looks in all purchase history and not just from the moment of starting the flow.

So at the end it is a preference thing and it depends on your post-purchase flow. If it is another long flow with lot of emails probably best to leave at least one of these filters so people won’t get to many emails but in case your post-purchase flow is short or you may not even have it than you can totally remove these filter and let people get all the emails not related if they buy or not. (in this case I would just suggest if you have discounts and reminders to add email filters to reminders in case people buy from the discount email to not receive the reminder so this would actually mean adding the flow filter Placed order zero times since starting this flow as email filter for the reminders.)