Hi, Zsuzsa,
Sorry for the late reply.
> Could you help me what is the difference between cron and action scheduler?
By default, Action Scheduler is initiated by WP-Cron, so there is not much difference. However, WooCommerce has a screen for monitoring action scheduler events in “WooCommerce > Status > Scheduled Actions”. To do that for standard WP cron, you would need to install an additional plugin, e.g., WP Crontrol.
> When he cron runs does it go thru every order?
Not exactly. The plugin goes only through orders with status matching one of the “Status from” options.
> Is it possible to run a rule not cron based, but on order status change hook based?
Yes. You need to go to “WooCommerce > Settings > Order Status Rules > Advanced”, disable both “Periodical Processing” options, and make sure that the “Rules processing hooks” option includes “Order status change”.
I hope this helps. Please let me know if you’ll have any questions.