Mail Propertries for Filter and Splits

Hey Klaviyo People! 

I’m pretty much stuck at creating a flow which splits by opening a specific mail inside the flow. I found inside the configuration of the split a filter for “where” which indicates that I could split the people by “Openend Mail” and equals something, but the problem is, inside the “where” I can put in whatever I want. There are no specific email properties which I can choose from, like subject or email name. 

So, question A: 

Is the where setting, a setting for email properties I can describe?

Question B: 

What are the email properties? I went through 20 help articles but I only found person properties, no email properties. 

Thankfull for any help!