COTW says it is a pruning group 2 clematis but also says it is a seed line of Helios which is definitely a type III clematis. I have grown Helios and treated it as a type III like I would for any other tangutica. Not sure why Sun Star could not be treated as such also. Curious about the listing on COTW though! If you do grow it, make sure to send a picture of it to COTW since they don't have a picture yet!

You can also visit the COTW website and send an email to Richard who maintains the website to ask about the pruning class issue with this clematis. He is always courteous and willing to help, but he does travel at times so be patient for a response.

Sorry I couldn't be of any more help!

Here is a link that might be useful: Sun Star