@Letitia Tita 

No this is not true. Browse abandonment and added to cart metrics only trigger when a Subscriber visit a produtct page or add the product to the cart. But if they are not a Klaviyo subscriber, if their email is not cookied by Klaviyo, Klaviyo is not able to track their activity.

For checkout it is a bit different, if someone who is not a subscriber starts a checkout with their email added into the checkout process and they have the accept marketing box checked then they will become subscriber with this action and at the same time they will trigger the checkout flow

Yes, but in your Active Profiles on your Klaviyo account you can have profiles that are not subscribed, and you still send them some sort of  flow emails. 
If you check under the Profiles tab you will see a certain number of profiles, but if you create a segment and choose Accepts Marketing equal true, you will see a different number. 
Correct me if I’m wrong. 

@Letitia Tita 

Yeap you are wrong again sorry.. :) 

Accept marketing in klaviyo is not meaning that those are the only people who are your subscribers. This is just one way of subscribing to klaviyo so those people who have accept marketing are simply people who subscribed in a different way from the others but all the profiles you have in your Klaviyo account are your subscribers and left their email somewhere on your website..