On what metrics the "Responded to Campaigns" segmentation filter work?

Hi Team,

I have created a test campaign, which contains an email template, which was then activated. I had received the email after activating the campaign, and also I replied back to the email as well. Then I used the segmentation filter "Responded to specific campaign" as shown below, but still the segment count is 0, even after I have replied to the email?

So, please let us know the basic concept of "Responded to campiagn" segmentation filter and on what metrics is the response of a campaign been calculated?

Thanks,

Akshara