The "Eval Method" on an Order Rule determines when the Order Rule will get processed (or reprocessed, it may get processed multiple times). It's really a performance setting. It shouldn't affect the end result of an order, it just determines when you see those results.
"Cart Changes" means the rule will get evaluated anytime the Cart changes. This is bad from a performance standpoint because it reprocesses the Order Rule any time the cart changes. However, it's good if you want to see the results of a rule in real time. Example: You would use this if you want the customer to see that they're getting Free Freight because their order was over $50.
"On Hold Status Changes" means the rule will get evaluated one time when Job Master reviews the order to take it off hold. This is good from a performance standpoint (since it only executes once), but bad if the rule is doing something the end user would want to see during the order creation process. Example: You would use this if you were placing the order "On Hold" if certain conditions were met. It would not be necessary for that rule to process during a "Cart Change".