Fiddler always captures anything that's sent to it. The (misleadingly named) "Capturing" setting controls whether Fiddler is registered as the system proxy. If traffic is sent to Fiddler while "Capturing" is off, that means that the client in question does not respect the system proxy setting and it sends traffic to Fiddler even when it is not the system proxy.
What is the client application? You can often prevent spamming of your session list by using the adjacent dropdown in the status bar to "Hide All" subsequent requests.
Fiddler always captures anything that's sent to it. The (misleadingly named) "Capturing" setting controls whether Fiddler is registered as the system proxy. If traffic is sent to Fiddler while "Capturing" is off, that means that the client in question does not respect the system proxy setting and it sends traffic to Fiddler even when it is not the system proxy.
What is the client application? You can often prevent spamming of your session list by using the adjacent dropdown in the status bar to "Hide All" subsequent requests.