When starting a campaign, does it send to everyone on the list or only newly added prospects?

I'm sorry if this is a dumb/obvious question, but I can't seem to find this answer in any of the engagement studio documentation.

I have an automation set up that matched with 35 prospects and added them to a "Trial Customers" list.

I have an engagement studio campaign set up to send to that Trial Customers list.

When I click to "Start" the campaign, does it start by adding all prospects currently on the list to the campaign AND any new ones that get added? Or does it only send to new prospects that get added after the time I click "start"?

I only want it to trigger for new prospects added to the Trial Customer list, since the emails won't really apply to the ones who are already on the list (I'm trying to test it first, but also because the timing will be wrong).

If it sends to the existing prospects too, I suppose I can add a filter step and use a tag or something to check if they were existing or not, wouldn't be too hard, but I'd like to know if I have to do that or not.

Thanks!

@* Marketing Cloud Engagement *