Use "%sc" as a condition when using set-map

Hello. I noticed that when I use “%sc” as a value to write to a map file, this is possible. However, if I try to use it as a condition, it does not seem to work. As an example, this is what I mean:

http-after-response set-map(/path/to/ %[srv_name] "%sc" if { status 503 }

The above works correctly. It writes “%sc” (number of server connections) to the map.

However, if I try to do something like below:

http-after-response set-map(/path/to/ %[srv_name] %[be_name] if { "%sc" 0 }

Then I get an error: condition : missing fetch method in ACL expression '%sc'.

How can I use this or create an acl to be able to use it in the manner I mentioned above? Thank you.