“You can’t re-distribute the Item as stock, in a tool or template, or with source files. You can’t do this with an Item either on its own or bundled with other items, and even if you modify the Item. You can’t re-distribute or make available the Item as-is or with superficial modifications. These things are not allowed even if the re-distribution is for Free.”
as a freelancer you can sell the customize theme to your client but for each sell you will need individual license.
If your customer website will chnarge for accessing their website or part of the website then will need extended license. If the website is full free of cost to get access then regular license is fine.
the main difference is that under the Regular License, your end product (incorporating the item you’ve licensed) is distributed for free, whereas under the Extended License your end product may be sold. Extended License is still limited to a single end product.