Usage of "newsletter" value under UTM source

Hey,

I understand this is not fully connected with SEO linking, but I cannot find better thread.

I am wondering why use “newsletter” value under UTM source, rather than actual source that is sending the newsletter, for example Marketo?

Which UTM tag do you use and what are they pros/cons

A. source=newsletter, medium=email, campaign=campaign_name
B. source=marketo, medium=email, campaign=Newsletter_campaign_name

Thank you