Welcome to the community @Kelvin 

You can use dynamic codes in flows and campaigns. The main difference is that for flows (with a Shopify integration), Klaviyo will automatically ensure you have sufficient codes for your flow emails. For campaigns, you must manually generate sufficient codes for the list/segment you are sending to.

The other difference is that dynamic codes for campaigns cannot have variable expiration dates i.e if you do want an expiration date, it must be the same for all codes.

More information here: https://help.klaviyo.com/hc/en-us/articles/115006155388 and here: https://help.klaviyo.com/hc/en-us/articles/115005084727

Hope that helps

Andy