When you add one source by means of the "Add" button or change one or several downloaded sources, you can see the "Source settings" window.
The "Proxy" tab includes the settings for checking proxies, downloaded from this source (or several sources).
"Proxy" tab description

Check options |
Select the desired options for checking proxies from this source. |
Proxy type |
Select a certain type of proxies, that will be used for a check. By default the type will be determined automatically |
Request timeout |
This parameter specifies a maximum response period after a check request |
Number of request attempts |
You can set several attempts for a proxy check request |
Pause between requests |
This parameter configures a pause between check requests, if their number is more than 1 |
Re-check restriction |
Time interval, when a proxy from this resource won't be accepted for a re-check, if it has been taken from a "live" list. (Selected with deletion) |
See also the description of the "Source" tab in the How to configure source parameters section.