List-trigged flows trigger a flow when they join a particular list or are added to any list. A welcome series is the typical list-triggered flow.
Subscribers can only receive a list-triggered flow once and only the first time they join a list. This means that if they were removed from the list and re-added, they won't be added to your flow
Creating a List-Triggered Flow
To create a list-triggered flow, create a new flow and select a list from the dropdown on the lefthand side of the flow editor.
You can also choose to exclude subscribers who are in another list or segment.
Once you select a list and save, you can no longer change the list that triggers the flow.
How a List-Triggered Flow Works
A list-triggered flow will add subscribers to your flow whenever they join a particular list or any list. For a welcome series you'd likely select "All Subscribers" or the list you have connected to your signup form or popup.
Subscribers can only receive a list-triggered flow once and only the first time they join a list. If they are removed and re-added to the lists they won't receive the flow. They also won't receive the flow if they've been on your list with a suppressed status and then become subscribed. For example a subscriber being unsubscribed on a list but their status changing to subscribed.
A subscriber will remain in the flow until they pass through all the steps to the exit, even if they are no longer in the list while within the flow. This is with the exception of subscribers who unsubscribe, bounce or change their subscription status on your connect platform (Shopify, BigCommerce etc)